The only Catalyst 4000 that can have dual Supervisors is the Catalyst 4507R.
If it is a new purchase, take a Catalyst 4500. The big advantage is that it has higher output power supplies which allow Inline power to be supplied without an external Power Entry Module - as previously required on the Catalyst 4000.
Catalyst 3550 is more intended for workgroups and is priced accordingly. Depends on the business requirement as to which is the best solution.