I’ll add that none of this is particularly cheap. Levelling a 4* hero costs 167,592 hxp.
At 9.72 hxp/gem, this is 17,242 gems needed. At a penny a gem, that’s $172 to level a 4* (assuming you have ascension mats).
5* heroes require 353,599 hxp. So that’s 36,379 gems, or $364.