Ben
I think you wuld be better off to use sme v5.6 as it is, the feature set is very good and it all works well despite objections from various users re not having a firewall in the same box as your server etc etc.
I would only suggest you use the 486 running v4.1.2 as a test box to learn about e-smith/sme if you really must, but to connect it to the Internet for production purposes would be dangerous (security wise).
Preferably ditch the 486 box and get yourself a low end pentium. Old pentiums are being trashed every day and are cheap or free to acquire. I have 5.6U4 running on a P133 with 64Mb RAM, but I would say that is absolute minimum spec, and it does run OK, although slowly.
Something like a Pent 333 (or P233) with 128 or 256Mb RAM would give quite reasonable performance as a full featured gateway, web & file server, I have set one up with 5.6U4 and it is running OK.
Regards
Ray