I think 4 is enough if they increased how much you get from them. It does not transition well at higher levels. When you first open it up you have groups at about 15k power and get like 150k gold and 40 xp pots in 10 hours. That 150k will get you like 15 levels in skills and 40xp pots looking at 2 to 3 levels from 1 mine. By 90 you only getting 280-300k gold witch will only buy you 1 maybe 2 skill levels 70-80 pots at most witch will not even bring a hero half way from 89 to 90. Mines become pretty useless at this point there more for the diamonds then anything else by then.