GTA San Andreas is not number 5, it has been rumored that it will have a lame co-op mode but it'd hardly be worth playing and it won't be online I am pretty sure..

Now back to what I was saying, GTA: Vice City and San Andreas are not GTA 4 and GTA 5, they are more like updated verions of GTA 3 then anything. GTA 4 will be a next gen title and since the exclusive rights with Sony will expire soon then my bet is that they will have a lot more freedom in being able to develope GTA 4 for all platforms rather then just the PlayStation franchise.