That depends on what you mean by half a hole. If a full hole is 64 cubic feet, then two men could dig a 32 cubic feet hole the dimensions of which are cube root of 32 in 5.5 hours. Assuming the four men worked as efficiently as two.

If by half a hole you mean a 2ft by 2ft by 2ft hole than that is only 8 cubic feet and they could do it in 1.375 hours. That is pretty slow digging. The ground must be very hard or the men very lazy.

2.75 hours for 2 men to dig the hole. Just out of curiosity,whats going in the hole?

You can't dig half a hole - no matter what size it is, it's still a whole hole!