At a consulting company I used to work at we defined our "core processes" and in a bizarre act of simple self insight - probably because it wasn't billable - they found we had two:
You're the system architect, are you the one doing the selling? Because I can't stress this enough, if you're not making sales you're going out of business fast. Even if you don't need a traditional salesman somebody has to promote the product in all sorts of media and get the word out to all your potential customers. The other part is having at least one guy who really groks code, since you're not it. You're going to produce a version 1.0 and it's going to have rough edges and it's going to have bugs. You won't have the to do all the things you'd like to do because you need to ship and make money, so stay on top of your early clients and make sure what bothers them is a top priority.
Is it a database-driven UI application? If so make sure you got database design experience as horrible table design and data inconsistencies will come back to haunt you, user interface designer who can also double as technical writer so your users actually understand to use it - this is also far harder than you think - in addition to the generic data processing skills. And really if that's three people, one salesman and if you haven't even started yet I wouldn't plan past that at the moment. If you're still alive and making money and looking to expand then you can start considering the rest. You'll quickly enough see where you need more people because you're out of resources, don't forget that the primary concern is running a business and secondary keeping your employees happy, if you fail at the first you fail.