I think perhaps you are thinking about 5's incorrectly. They are strong, but they must be used defensively more than offensively. 4's are better offensively as you mentioned. Any 4 can attack literally any non-bomb piece if it is backed up by a sapper. Why? if the enemy 5 recaptures it, the sapper is there to take it off the board. teams of sapper + 4 and spy + 5 work well together, as the spy identifies the potential sappers lying in wait.