Adding RAM will in most cases speed up your computer.
Unfortunately, you didn't provide us with enough information to give you an estimate of how faster will your computer be. If your computer is limited mainly by amount of RAM, you'll see considerable speed improvement. If you have more than enough RAM, you probably won't notice any improvement.
Amount of RAM is in a very indirect way related to hard disk space. Your hard disk needs to be large enough to be able to store enough data to actually fill RAM. As long as you don't use 10 years old hard drives in brand new computer, you won't have any problems with that.
Motherboard is the main limiting factor when determining amount of RAM which can be installed. In it's manual (or BIOS release notes) it will say how much RAM can be installed.
Processor does limit amount of RAM but in an indirect way. Basically your motherboard will be made in such way that it does not support more RAM than processors compatible with it can support. In some systems with very large amounts of RAM and multiple processor sockets, there may be cases where some empty sockets need to be populated before system will accept new RAM, but that's only a problem on expensive servers. As a user, you probably won't see such systems.
I don't know what you mean by Virtual Cache Memory. There are various caches in various devices and there is virtual memory (or page file or swap space, name depends on operating system).
Since you have 4 GiB of RAM, you may get improved performance with more RAM, but the improvement will not be large. 4 GiB should be enough for all common tasks such as browsing INTERNET, watching movies, playing games and similar. If you plan to run virtual machines or do some heavy programming, you could add some more RAM.
As far as cache is concerned: It is generally not related to amount of RAM. Processors, hard disk drives, optical disk drives and similar components use cache memory to store data they need in order to decrease number of accesses to RAM. Cache memories are more expensive (especially those used on processors) and on modern computers can't be upgraded.
As for virtual memory, it takes up hard disk space on windows and uses separate partition on many unix-like systems. You'll need to have at least as much virtual memory as RAM in order for everything to work correctly. There's no need to be particularly concerned with virtual memory when buying RAM.