As mentioned, the best bet that can at least guarantee successful running of four drill motors is the Electronize range. Specifically you'd be wanting two of the 30A units that can be modified by having their internal relays replaced with some bigger external relays rated at 40A which helps ensure that your controllers are less likely to be damaged from being over-stressed.

Technobots used to offer this product (the speed controller with a pair of external relays and any modifications necessary) but due to lack of demand they do not stock them anymore. You would have to contact Electronize directly (http://www.electronize.com) and ask if they could supply you with modified controllers. They don't provide the larger 40A relays though, you would have to buy them yourself. You can get them from Maplin or Technobots:

http://www.technobots.co.uk/acatalog/On ... s_360.html

(the 12V ones, and you would need two per controller, so four in total for full robot drive)

I will admit though, depending on how tight your budget is, that this option isn't necessarily cheap. I believe you'd be looking in the region of £100 for both controllers and relays. But Christmas is coming up, so let Santa know what you're after :wink: