The main difference, besides the 4 user limit, is you can't do a "session_connect" call which would allow you to move a user from one game server to another (called zones).
So for the Commercial vs Professional, yes everything is basically the same. Any code that you write now will work perfect in the professional version with the added availability of zones and unlimited players.
Keep in mind that unlimited players doesn't mean click together a game and have it capable of handling 600 users at a time... You will need to dig in and optimize your game to handle how many players you want in each "zone".
If you are just going for an 8 or 16 multiplayer game then you should be absolutely fine working with commercial and then upgrading when you are ready to compile with pro for the added users.
What type of game are you wanting to develop? Just curious. Not looking for details, just something like FPS, RPG?
Oh yeah, almost forgot there are other differences between Com and Pro such as mirrors, physics etc... but I assumed you were asking specifically about Multiplayer. I am sure you also know that you can visit the main page to see all the differences between editions, but just incase:
http://www.conitec.net/a4info.htmGood Luck!
