[2gb user + 2gb kernel]
That is a constraint by the OS. On an x86 32-bit system without PAE enabled, the virtual address space is 4 GiB (note that GB usually denotes 1000 MB while GiB stands for 1024 MiB).
What confuses me is that is this 4gb space unique for every process .
Yes, every process has its own 4 GiB virtual address space.
if I have say 3 processes p1 ,p2 ,p3 running would I need 12 gb of
space on the hard disk ?
No. With three processes, they can occupy a maximum of 12 GiB of storage. Whether that's primary or secondary storage is left to the kernel (primary is preferred, of course). So, you'd need your primary memory size + some secondary storage space to be at least 12 GiB to contain all three processes if all those processes really occupied the full range of 4 GiB, which is pretty unlikely to happen.
Also if say I have 2gb ram on a 32 bit system ,how will it manage to
handle a process which needs 4gb ?[through the paging file ] ?
Yes, in a way. You mean the right thing, but the "paging file" is just an implementation detail. It is used by Windows, but Linux, for example, uses a seperate swap partition instead. So, to be technically correct, "secondary storage (a HDD, for example) is needed to store the remaining 2 GiB of the process" would be right.
