Can I mix different RAM modules in PC?

Possibly.

However, having paired modules (e.g. 2x2GB) will more than likely give you better performance as the memory can run in Dual Channel mode rather than Single Channel. You won't get exactly double the transfer rate as there are overheads involved but it should be close.

The relevant thing to bear in mind is:

Modules rated at different speeds can be run in dual-channel mode, although the motherboard will then run all memory modules at the speed of the slowest module.

Probably, with some caveats.

All the RAM will run at the timing and latency of the worst stick of RAM there. You may also need to check if your BIOS/EFI supports that RAM size - some older versions may not support larger RAM sizes, and may need an update, and the total amount of RAM may be limited in the same fashion.