Well... I'm going to go out on a limb here. Some people will probably disagree with me, but what else is new?
Theoretically, you shouldn't mix brake pads front vs rear, just like you shouldn't mix tires front vs rear. The car is set up to be balanced with the same type of pads at both ends. However, as a
practical matter, the car will still function if you mix them - and this means you will need to keep in mind that you may have some imbalance front vs rear. If you have better braking at one end, this creates the possibility that that end may lock up the tires sooner than the other end (keep the ABS on!) and/or that one end may suffer overheating more than the other. If the types of pads are similar in characteristics, the imbalance may be minor, but if they are dramatically different - as track pads are from street pads - the problems can be serious. It's all stuff to keep in mind before proceeding.
Now, as for your specific situation... What is your intention? The Porterfield R4S is a street pad, and the Hawk Black is (I believe) a track pad. The fact that you're moving to the Hawk makes me suspect that you'd like to try track pads - and, as is typical with track pads, you intend to swap them for the track, and not use them on the street. Now, if this is the case - why wouldn't you want to get a set of the Hawk pads for both ends to use on the track? You'll still be able to use the Porterfields after you swap back to go home, no? So why would you be mixing the pads?