The more you train, the better you get. But since 4-way is a team sport, you have to keep a team together. As someone else said, it all depends on the level of commitment your TEAM has. Some common training measures are a) One weekend per month, b) every other saturday and c) every Sunday. For a new team, any of those should work. But Make sure eveeryone agrees to it.
I prefer to train every 6 jump every Saturday. My team agreed to train just one saturday last season, four jumps. So we competed in the meets with no training. We had fun, but didn't learn as much as I wanted. Needless to say, I'm looking for a new team this year.
Cost is big factor, especially with jump prices at $20! Video with a good debrief and/or coaching are well worth the expense. If your team is serious about becoming better skydivers, factor those cost in to you equation. If you get lucky, you can find a camera person who jump with you for free...but generally expect to pay for his/her slot. If you can't get free video, make fewer jumps and spend more time on the creepers...creeping free! You'd be amazed at how much time the good teams spend on the creepers.
You might get more team info from the Relative Work forum.