Decision and Control unit AAT level 4

My recommendation would be to stick to the same course provider material which you've been using.

If you're doing Kaplan for the rest of the assessments, use its materials for MDCL as well. The similarity would help you as quite a bit of MDCL links from L3 MMAC or to L4 MABU.

I personally used Osborne books. I've seen quite a few colleagues and friends use them. Osborne are simple, concise, and fluent. Not to say that other's aren't.

Then for anything else I dipped in and out of Kaplan's Knowledge Bank. Most of it is outside and ahead of AAT L4 but it links to L4 concepts very well and helps understanding by showing where things are going. As an example, their page on variance analysis is excellent short summary to help you along for L4 MDCL once you're clear on the concepts.

Don't forget to go through all the official AAT resoruces avaialble at their Learning Portal.