If you can buy drainable pond then it's better than undrainable pond. Why?
Advantages:
1) possibility to change water level. For example you can keep shallow water all the summer for the fish to grow faster. When autumn comes, you simply raise the water level and fish have a deep pond for winter.
2) it's 100% drainable. In case you wan't to get rid of fish or you find a leak or you want to clean some weeds (whatever) then drain it and do whatever has to be done. I doubt that you can use simple water pump for 4 acre pond
Disadvantages:
1) probably such ponds aren't not that common as undrainable pond;
2) they might be more expensive.
