As a fourth year I've been looking into this question and apparently there are many considerations. First it would be wise to see what kind of educational resources your residency will provide. Many institutions have online access to Harrison's (accessmedicine), Cecil's (mdconsult), CMDT (accessmedicine) and uptodate. These are all expensive and arguably more useful on a case-by-case basis so it might be beneficial to wait.
Options for reviewing topics in medicine before residency would then include those used during medical school: Step-Up to Medicine, Medicine Secrets, baby Harrison's/Cecil's, etc that many American grads use for their exams in internal medicine. Since you may be familiar with these already they will stir memories of past studies.
Another option would be looking though the ABIM review texts, including MedStudy and the Mayo review of internal medicine. These will likely have to be purchased during residency anyway and are supposedly comprehensive.
The main source for questions has traditionally been MKSAP, but there are many, many sources for questions out there.
There are so many options out there that it is hard to give advice without knowing you personally, and those who are currently residents probably have more to add. My plan is to a Sub-I in medicine towards the end of fourth year while going through the Mayo Review of Internal Medicine. I will save Harrison's, Medstudy, and MKSAP for residency.
