Since these two games were fairly similar (all the NFS3 tracks are in NFS4, plus NFS4 should be easier to install on modern computers since NFS3 doesn't support 64-bit) I'll combine these two together. Play either one or both - but make sure you participate and post pics/videos and rate the game.
NFS4 iirc also supported modding and with an extensive mod still supports online multiplayer. I think Andreaz is more up to date with all that jazz so I'll hand the mic over to him to post links and tutorials.