I was trying to find the verses referenced by SgtRock.
I assume when he was quoting "Quran 4:90" or "4:91" that he was referring to the fourth Sura, which is Surah an-Nisa yes?
4:90 and 4:91...
Neither of which appears to be what he was quoting, even accounting for a different translation.
However, 4:88-89 (Surah an-Nisa 88-89) are much closer:
That does seem to be saying that those who leave Islam should be killed. However, I don't claim to be an expert on the Quran, though I have read a lot of it and about it; if someone has an alternate interpretation, I'm listening.
The Holy Quran online
G.