It's going to depend on why your checks aren't purging. Is there something stuck or has the purge just not been running? Micros only gets rid of 10 days at a time, so if it hasn't been running you just have a lot of catching up to do.
If you're ok with just clearing totals your best bet is to do it overnight. 4G is huge for a Micros database, the largest one I have is 1.1G and that's in Times Square with a tremendous transaction volume.
You're going to have to run a rebuild once you're done purging or clearing totals. Those will null out the tables but not necessarily delete all the lines. Running a rebuild will kill the bloat in your tables.