My take on this: I do think in all probability you will cause yourself far more harm than gain by spending 4 minutes on (as in your scenario) each of 4 questions. That'd take up 16 of your 75 minutes, leaving you with only 59 minutes to devote to your other 33 questions -- and at that point any question you spend more than about 1 minute and 45 seconds on will drive you further behind. Also remember that you have a one in five chance of getting any given question RIGHT just by pure guessing, so the most likely outcome if you guess on 4 questions is that you'll get exactly one of them right. (There, see, you've already knocked it down to 3 wrong .)

Of course, you should TOTALLY spend 3-4 minutes on problems you think you could get right given the extra time IF -- and ONLY if -- you otherwise tend to run way ahead on time. As long as you've got time to spare, go ahead and use it. But otherwise, you'll probably wind up putting yourself sufficiently behind by dawdling on those 4 questions that you'll wind up just having to guess on 4 questions at the end! And in that case, it would certainly have been better in retrospect to guess on the ones you were shakier on from earlier than to guess on these random 4 end questions that might otherwise be totally within your ability to do quickly.