On Intel hardware, 512 meg of ram should be sufficient. 256 would probably do on a Sparc.
There are no hard/fast rules, since the more "hits" you start to get, the more CPU you are going to need. But 4,000 a day translates into only a few million over all hits per month, which is nothing to a Unix machine.
You might want to consider dual SCSI harddrives, putting the OS and logging on one drive, and the web and user accounts on the other. You'll gain a lot of I/O performance with SCSI over EIDE especially for webserving.
Your big process will be "rebuilding" the site. Right now it takes almost 2 hours to do a full rebuild for about 40,000 links and 3,000 categories with detail pages (but I do 3-4 "includes" per page which I _know_ has to slow things down). Without the detail pages it's under 45 minutes depending on the load to the rest of the system.
http://www.postcards.com FAQ:
http://www.postcards.com/FAQ/LinkSQL/