Minor 4 chords

[Plain G]

A couple songs I know (Across the universe and Valentines day) employ the minor 4 chord as well as the major 4. It sounds good but why? Is there a theory thing behind this?

edit: Can't think of a song when the 5 chord goes minor, is that a no-no?

[Fendershredder]

I assume you're talking about in a major key.

The iv chord resolves really nicely to the I chord because it gives you some half step motion back to the I.

In C:

iv=F Ab C
I=C E G

F---->E
Ab--->G
C---->C

All half step resolutions, and a common tone. You also get the satisfaction of hearing the chord move in a 4th/5th motion, which always helps. I love the iv chord in a major key.

[Plain G]

Very cool. Thanks :) ... and you were right to assume I meant in a major key