The math irony. 1.5^4 ~= 5.0
Still I think the long term hashrate growth will be more than 50% but your statement actually confirms the OP claim.
It 's no irony or coincidence obviously. It's what i calculated but it seems that i expressed myself poorly

I meant for the difficulty to be 5 times greater it takes only a 50% rise per month for the next 4 months. 50% per month is very low compared with what we are witnessing in the past few months.