Why would you recommend levels 1+2 over the harder levels though?
Short version: The main reasons are timing, endurance, and getting used to seeing many different question types, and ranges of difficulty in a row.
I guess that suggestion requires a certain number of assumptions, one of which is that if you don't have trouble with the ones and twos then you are capable of acing the threes and fours. And that if you learn from your mistakes in the ones and twos, then you don't have trouble with them. There are more, but this isn't an LR question, so I'm done.
Basically if you do all the threes and fours, then the sections those questions come from are useless because you will have done them before, and are likely to remember at least some of them. However, if you only do the ones and twos, then you can still use the old sections as additional sections in future PTs to increase endurance. In addition, you can continue to learn from your mistakes, since you haven't seen the difficult questions before. Now I will concede that a certain number, I have no idea what this number is, of level three and four problems could be beneficial if you are struggling with a particular question type. However, referring back to my assumption, if you are good with the ones and twos, then you should be able to tackle the threes and fours.