Either way, 4 years is doable if you gradually lower your expectations of performance and quality as I've stated. The problem here is that such a PC can cost quite a bit I'm afraid. You really will want the last of the last and fastest PC parts.
You can upgrade but you cannot upgrade forever. CPU sockets are often getting updated every CPU generation and if lucky will last you 2 generations. PCI-E socket (the stuff for the GPU) is a bit more lenient but wait too long and you will be bottlenecked by either CPU or PCI-E speed (though they are barely hitting that last I've heard).