If you're coding in PHP then it is extremely easy to switch IP
addresses in your scripts. You simply set the IP address from where
the call is made with CURLOPT_INTERFACE, provided that your different
IP addresses are on the same server.
There are two ways you can control your 20,000 per hour rate limit:
a) You can segment your GETs and do, for example, all /users/ calls
from one IP address, all /friends/ and /followers/ from another IP
address, etc. No need to worry about POSTs because they don't count
against your rate limit.
b) You can examine the X-RateLimit-Remaining header of each call, and
dynamically switch your scripts over to the next IP address as soon as
that number approaches zero.