Most schools require the deposit to be postmarked by the set date. You should check on that, because it may allow you some more leeway with when you need to submit a deposit.
Also, you need to ask yourself: is one (or more) of the 8 schools somewhere you would definitely go to, over your 4 accepts/4 waitlists? If you want to hold out for other schools, pick your favorite of your 4 accepts, put down a deposit there, and ride the waitlists/slow decision-makers. You might also try asking your preferred school for an extension on the deposit date, but I have no idea if they're friendly to that.
So, the bottom line is: if you're intent on trying to attend a school you haven't heard from yet, you may need to sacrifice the deposit to a school you've already been accepted to. You can make a deposit and later choose not to go.
Out of curiosity, what schools are these? Do you have an LSN?