Do you know who made your tank? Tempered panels sometimes have little paper notices saying they are tempered. Most likly if a tank does have tempered glass its only the bottom so drilling the back is ok. If its an all glass aquariums tank then you can go on their website and see if its tempered bottom.
Size the holes based upon the bulkhead you need. For example a 1" PVC
bulkhead might require a 1 3/4" hole to be drilled. SO first figure out what size plumbing your going to be putting thru the tank and then get the proper bulkhead for that and it will say how big a hole you need.
The concept for 4 holes is two returns (the ones on the ends) and two drains (the ones inthe middle). The two drains will provide redundancy for the most part where as the two returns will maximize water flow.
Use at a minimum 1" plumbing for your drains. 1 1/2" plumbing would be better. Size your returns based upon the pump your using.