This stuff is dangerous, if you don't know what you're doing you should hire an electrician. You can't take your bitcoins with you after you electrocute yourself.

I explain 240v circuits in the USA here: https://bitcointalk.org/index.php?topic=2772135.msg28578662#msg28578662

The 4 prong dryer is 2 hots, a neutral and a ground. It could be rewired to provide 2x 120v circuits (if the shared neutral awg is large enough), but may not be up to code if the breaker is too large. A lot of these dryer breakers are like 50 amps. You'd need to swap out the breaker at minimum for a lower rated one to stay in code. The best would be to swap it out for a 30A 2 pole breaker, use the red, black and green wire and use the L6-30R outlet. You'd end up with 240V and 30A of power (5.76kW continuously usable). The white wire you'd want to disconnect at both ends and tape / label where it goes.

Note: I am not an electrician. I know enough to know when I don't know something (could probably pass a journeyman test). Which is only slightly better than knowing enough to be dangerous.