forgot about ya lastnight sorry lol...
First, Nice tank and even nicer Dad you have, you should be very thankful lol
Second, don't be nervous its not in very bad condition at all. Its a good thing most of the separation/crazing is at the top back of the tank and not along the upright seams or bottom. Pics are a bit hard to decipher but it kinda just looks like stress crazing which is better than seam separation. Since you have the #4, you can try shooting it in the seams where you see white color and try to seal up that bond again. If it becomes clear again on the seam your doing the right thing. This can be tricky on repairs and may not work for all the spots but itell get you familiar with working on the tank and you cant hurt anything by trying the #4 first. This is not a permenant fix but the first step. Second step would be to tilt the tank at a 45 degree angle and work on one seam at a time where you see white spots with a thicker type glue like the #16 you have. #16 is not the greatest and I usually don't recommend it but it holds up for a couple years. I do not think you need a full overhaul with the tip and pour method and #40 as the tank looks pretty new. With the #16 you can add in a piece of 1/2" square stock acrylic inside the tank to beef up that area of separation. You can really do as much or as little as you want, all depends on how much you want to fix it. If it were me id overhaul the whole thing with #40, but I have the glue on hand at all times and have done a lot of tanks so im farmiliar with it. Not sure if ur parents will appreciate you blasting them out with 2 part epoxy lol...
Let me know what your ideas are after reading and I can further help you with what way you want to fix it or help during the process. Good luck! repairing tanks can be fun lol