Chapter 3 ended with an angel telling Nephi's brothers to stop beating Nephi and Sam with a rod.
And it came to pass as they smote us with a rod, behold, an angel of the Lord came and stood before them, and he spake unto them, saying: Why do ye smite your younger brother with a rod? 1 Nephi 3:29
The angel didn't impress Laman and Lemeul much though, so Nephi had to make another long, boring, BoM-type speech.
And it came to pass that I spake unto my brethren, saying: Let us go up again unto Jerusalem, and let us be faithful in keeping the commandments of the Lord; for behold he is mightier than all the earth, then why not mightier than Laban and his fifty, yea, or even than his tens of thousands? Therefore let us go up; let us be strong like unto Moses; for he truly spake unto the waters of the Red Sea and they divided hither and thither, and our fathers came through, out of captivity, on dry ground, and the armies of Pharaoh did follow and were drowned in the waters of the Red Sea. Now behold ye know that this is true; and ye also know that an angel hath spoken unto you; wherefore can ye doubt? Let us go up; the Lord is able to deliver us, even as our fathers, and to destroy Laban, even as the Egyptians. 1 Nephi 4:1-3
After the speech, Laman and Lemuel decided to follow Nephi back to Jerusalem, though they "did still continue to murmur."
Now when I had spoken these words, they were yet wroth, and did still continue to murmur; nevertheless they did follow me up until we came without the walls of Jerusalem. 1 Nephi 4:4
When they got to Jerusalem, Nephi's brothers waited outside the gates while Nephi was led by the Lord to Laban's house.
And it was by night; and I caused that they should hide themselves without the walls. And after they had hid themselves, I, Nephi, crept into the city and went forth towards the house of Laban. And I was led by the Spirit, not knowing beforehand the things which I should do. 1 Nephi 4:5-6
The Lord lead Nephi to a drunk guy passed out on the ground -- who turns out to be Laban!
Nevertheless I went forth, and as I came near unto the house of Laban I beheld a man, and he had fallen to the earth before me, for he was drunken with wine. And when I came to him I found that it was Laban. 1 Nephi 4:7-8
Laban had a sword made out of "the finest steel" (more than two thousand years before steel was available) with a handle of pure gold.
And I beheld his sword, and I drew it forth from the sheath thereof; and the hilt thereof was of pure gold, and the workmanship thereof was exceedingly fine, and I saw that the blade thereof was of the most precious steel. 1 Nephi 4:9
Then the Spirit constrained Nephi to kill Laman, who was passed out on the ground in front of him.
And it came to pass that I was constrained by the Spirit that I should kill Laban. 1 Nephi 4:10
But the Spirit had to tell Nephi again.
And the Spirit said unto me again: Behold the Lord hath delivered him into thy hands. 1 Nephi 4:11
And a third time, reminding Nephi that God kills people too (so it must be OK) and besides, it's better that one person die than a whole nation dwindle in unbelief.
And it came to pass that the Spirit said unto me again: Slay him, for the Lord hath delivered him into thy hands ... It is better that one man should perish than that a nation should dwindle and perish in unbelief. 1 Nephi 4:12-13
So Nephi (finally) obeys the Spirit by grabbing Laban by the hair and chopping off his head with his own sword.
Therefore I did obey the voice of the Spirit, and took Laban by the hair of the head, and I smote off his head with his own sword. 1 Nephi 4:18
After Nephi smote off his head, he put on Laban's clothes (while the blood gushed from the carotid artery) and "gird on his armor about [his] loins." (Laban always wore armor on his loins when he went out partying.)
And after I had smitten off his head with his own sword, I took the garments of Laban and put them upon mine own body; yea, even every whit; and I did gird on his armor about my loins. 1 Nephi 4:19
Dressed in Laban's blood-drenched clothes and loin armor, he went to the treasury of Laban and commanded (with the voice of Laban) Laban's servant to follow him.
And after I had done this, I went forth unto the treasury of Laban. And as I went forth towards the treasury of Laban, behold, I saw the servant of Laban who had the keys of the treasury. And I commanded him in the voice of Laban, that he should go with me into the treasury. 1 Nephi 4:20
And it worked perfectly. Nephi looked and sounded just like Laban, so it completely fooled Laban's servant.
And he supposed me to be his master, Laban, for he beheld the garments and also the sword girded about my loins. 1 Nephi 4:21
Nephi chatted with Laban's servant for a while about all the usual things -- the wild party last night, gossip about the elders of the "church" (the Jews had churches back then), etc. Then Nephi told him to take the brass plates to his brothers who were waiting outside the city gates.
And he spake unto me concerning the elders of the Jews, he knowing that his master, Laban, had been out by night among them. And I also spake unto him that I should carry the engravings, which were upon the plates of brass, to my elder brethren, who were without the walls. And I spake unto him as if it had been Laban. And I also bade him that he should follow me. And he, supposing that I spake of the brethren of the church, and that I was truly that Laban whom I had slain, wherefore he did follow me. And he spake unto me many times concerning the elders of the Jews, as I went forth unto my brethren, who were without the walls. 1 Nephi 4:22-27
When Nephi's brothers saw Nephi and Laban's servant coming, they were "exceedingly frightened" since they thought Nephi was Laban, too! (It was an exceedingly good disguise.)
And it came to pass that when Laman saw me he was exceedingly frightened, and also Lemuel and Sam. And they fled from before my presence; for they supposed it was Laban, and that he had slain me and had sought to take away their lives also. 1 Nephi 4:28
But then they heard his voice (he wasn't using the voice of Laban this time) and they knew it was Nephi "wherefore they did cease to flee from [his] presence."
And it came to pass that I called after them, and they did hear me; wherefore they did cease to flee from my presence. 1 Nephi 4:29
After brothers settled down, Nephi talked Laban's servant (Zoram) into joining up with them, and they packed up the brass plates and returned (400+ kilometers) to Lephi's tent.
And it came to pass that we took the plates of brass and the servant of Laban, and departed into the wilderness, and journeyed unto the tent of our father. 1 Nephi 4:38