Before i begin, i should point out that this guide is really for PC enthusiasts or "power users" (for lack of a better term). If you have even the remotest doubt that you would need as much as 4gb of memory in your PC, then that almost certainly means you won't need it, so you may as well skip the rest of this. For those considering buying 4gb, or upgrading to that amount, read on....
This is not so much a guide to help in choosing a particular brand, type or speed of PC memory (RAM), as i'm sure you're all capable of finding memory that suits your budget without having to read a guide on it (there are several guides on those matters already if you do need one though), this is more of an outline into the do's and don't's of buying 4gb of memory for your PC, because once you decide to install such a large amount of memory, you may find yourself with more problems than you bargained for.
This first thing to point out is that unless you have a 64-bit Operating System (Windows XP x64, Windows Vista (Home Basic, Home Premium, Business or Ultimate) 64-bit or one of the 64-bit Linux distributions, you CANNOT use 4gb of memory! This is nothing to do with a hardware incompatibility, or a problem with Windows XP Home/Pro or earlier versions of Windows, this is entirely down to a limitation of 32-bit computing.
32-bit computers can only access a maximum of 4gb of memory in total, which not only covers your RAM, but also you video cards' onboard memory and in some cases sound card onboard memory. For example, if you had a high end PC with a brand new Radeon HD 3870 X2 video card featuring 1gb of GDDR4 video RAM (Note: 1gb = 1024mb), and a Creative Labs X-Fi sound card with 64mb of X-RAM, this makes a total of 1088mb of memory that the Operating System is addressing already. If you then install 4gb of memory into your PC, the 32-bit Operating System will essentially ignore 1088mb of that 4gb (4096mb), and will only recognise 3008mb of system memory, which is a shade under 3gb. This means you will have purchased 1gb that not only never gets used, but as far as your PC is concerned, isn't even there!
This can go on to cause other problems, if wasting money isn't a big enough one, especially with modern motherboards that use dual-channel RAM. The motherboard will almost certainly at some point need to send data to the part of the RAM which is effectively cut off by the Operating Systems' inability to detect and use it, which will result in system crashes, lost data, and possibly worse. Eventually Microsoft brought out an update for Windows XP and Vista (the only 32-bit versions they still support) which limits them to only supporting 3.5gb of system memory, regardless of what other devices that have RAM fitted are installed. This solved many of the problems of possible data loss but does nothing for those people who buy 4gb RAM to use with their 32-bit Operating System.
Don't think that i am saying 4gb RAM is a waste of time and money, those of you with 64-bit PC's, particularly if you are using Vista, will find that 4gb really is the sweet spot with your Operating System. Not only can 64-bit Operating Systems support 4gb with no issues (they can actually support anywhere from 16gb up to 128gb), but you'll notice a good performance improvement even over 2gb. Whether you would need more than 4gb is debateable, unless you are doing some professional level animation of film editing perhaps, but for now 4gb is a good companion with a 64-bit Operating System.... Just be sure you have a 64-bit Operating System before you buy such an amount.
Update: February 27th 2008
The release of SP1 for Windows Vista brings a change to the RAM detection in all 32-bit versions. Now they will show all installed system RAM, so if you have 4gb installed on your motherboard, it will show in the 32-bit versions. However, it will still only be able to use up to 3.5gb of it, because of the limitation of 32-bit computing, so in my opinion this change will only add to the confusion.