Chaim,
We use T/S 4, two things to look out for, 1 is that MS requires a NT 4 workstation license for each access to the server, even if the client is win98 or other, you must buy a NT 4 license (this was not true on the Citrix version)
The other is the "Metaframe" add-on, which allows clients other than NT 4 to access the server, including Win9x, DOS, OS2 etc. Without purchasing metaframe, your T/S clients must be running NT 4

HTH,
Ed