The problem is that by design, an SQL Server will take as much memory as it can (the larger it's cache the better it's performance will be) and as such, running 4 SQL Servers will simply fight for your ram (1gig is no where near enough.)
That said, the maximum database size is 4 gig, but unless your dealing with images you'll be surprised just how much that is.
We used SQL express and it was fine for what we were using it for (10 user client/server application), plenty of SQL and plenty of queries every second. I would recommend using the express version, and looking into a better one in the future.
FYI: Microsoft Small business server (SBS) comes with a workstation version of SQL Server, plus exchange which means it is EXTREMELY cost effective for a starting business.
Also if your idea is good, you should look at the microsoft programs for new companies (It used to be called empower, it has a new name now...) they are really really good in terms of software, especially server software.