So, now is it wise to use XP 64Bits?
Yes - Except that I am of the opinion that if you're going to make the jump to 64 bit computing, you'd be better served going to Vista 64 rather than XP 64, because the newer OS now has better/further development and support, and will continue to enjoy that going forward. Whereas XP64 development has ceased, and only patches for Bugs/security will be forthcoming from MSFT.
1] If I use x32 with 4gb Ram, will I get any cons, other than not detecting full amount?
Nope - System stuff gets addressed first, then RAM, and whatever diesn't get an address is merely ignored. Unless you're looking in the management concsole you wouldn't notice much difference.
2] If I use x64 is their any cons? (I use a lot of photoshop etc..). Also do I need to change all my existing software’s to x64 versions?
No - x86 software runs just fine. The exception being if you have really old apps/games with 16 bit code or 16 bit installers.
The fundamental reason why is because in a binary level X64 is a "Superset" of X86. What that means is all the X86 instructions are included in the X64 spec - Just there are more available in X64, obviously. (And also 64 bit has the capacity to run/move larger messages per clock cycle) So as long as you have an X64 capable processor, the binaries will run unsullied and untranslated and there will be no performance impact.
On the other side of that, what matters is that the applications were code to use the proper API's for the operating system - And the Window's API hasn't significantly changed, except that drivers and apps are no longer permitted to use Kernel mode commands. For most apps it's a non-issue, but you will need to check that your drivers are compatible.
Regarding Photoshop - You should check on this. I do not believe that Adobe is supports Photoshop in XP64. The newest versions do support Vista 64.
3] In x32 say my GPU is 1 GB Memory and my Ram is 4 GB – Will I only get 3 GB of Ram detected?
The short answer is 'Not Quite" - The system itself still requires 500~750MB for itself (Bios, Chipset, and various devices/communication). So with a 1GB card, you may find yourself limited to 2.25~2.5GB of RAM, or so.
4] All Games old/new play on x64 edition?
As already mentioned - So long as you're not running 16 bit code, or games with a 16 bit installer you should be fine.