There is a version of GetIplayer for PC but I do not think it has been updated for quite a long time and only seems to work with BBC. The iPlayer iPad app already supports downloading of most things, so would only be useful for things that were rights restricted like films.
If Mac, my favourite software at the moment is Get iPlayer Automator. The beauty is that it downloads from the four main channels, not just BBC and it automatically sticks it into iTunes ready to copy to your iPad. Tis brilliant
Some things from 4OD are on youtube and there are various youtube downloaders around which you could try, but may be risky as hard to know what those bits of software will do. Then once downloaded, if it works, you then need to transfer the file to the ipad, plus you may need to find a player that will play the file. All a bit fiddly.
Don't suppose anyone can recommend something they know works and is mallware free, easy to install etc?