This is seriously entering the realm of speculation, but GTA4 sales can be projected using the above examples (cementing this as a less-than-reliable technique, I might add), by assuming that the next GTA will arrive within 2 years, and that the userbases of the 360 and PS3 will grow according to the general sales curve they're following.
Scenario 1: GTA3 sales levels
GTA4 ~ 11.90m on PS3
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 35,000,000 (~34% of userbase)
GTA4 ~ 10.88m on 360
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 32,000,000 (~34% of userbase)
Cross-Platform Sales ~ 22.78m
Scenario 2: GTA:VC sales levels
GTA4 ~ 7.00m on PS3
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 35,000,000 (~20% of userbase)
GTA4 ~ 6.40m on 360
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 32,000,000 (~20% of userbase)
Cross-Platform Sales ~ 13.40m
Scenario 3: GTA:SA sales levels
GTA4 ~ 4.55m on PS3
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 35,000,000 (~13% of userbase)
GTA4 ~ 4.16m on 360
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 32,000,000 (~13% of userbase)
Cross-Platform Sales ~ 8.71m
As a projection, this is largely like shooting in the dark in the general direction you think your target might be in. So please, whatever you do, do NOT take this as an authentic prediction or even remotely reliable estimate.
Sky Render - Sanity is for the weak.
