IV for me.
Been six years and I missed this thread? How? Dunno.
Anyway, I do play V more than IV(BoGT), but IV feels better in terms of the landscape. Being able to travel to the different boroughs. I miss that part of NYC(I don't miss the cold weather though).
V feels like I'm just inside one big circle. A Petri dish.
For all V's improvements, it lacks the functionality of IV.
I mean, why own all these clubs in V, if I can't enter them? I have one girlfriend as Franklin, a hook up as Trevor and a family as Michael. Yet, I can't drive them all in one car and go to the mountains or on a boat ride. Can't take my gf to the amusement park(well, she is a bit out there), etc.
Now, the mechanics of IV, are horrible. That's the main reason I play V more than IV. I still like the storyline of IV, TLD & BoGT. At least, the journey I immersed myself in.
Lastly, the soundtracks. I call IV... by a hair. Just by the era, I stay tuned to VLADIVOSTOK, TUFF*GONG and electroshock.
With V, it's listening to Pam Grier, Bootsie Collins and FlyLo and only A.D.H.D, when it plays.