Well played Kabam! Act 4 seems like it's the game's best cash-grab.
Just to provide context, I am currently at level 52, having completed up to Act 4.3 (hoping to complete full Act 4 by next week) and I have fully explored 4.1.
Difficuly is certainly not a problem with completing / exploring Act 4. For completing 4.4 I will use my A-team (now that War season is over) and I have been exploring with a mix team of 4* 3/30 & 4/40, without using any revives / potions / boosts (just the way I like it!). But, by exploring through Act 4 you have to face the following:
1) the extremely long routes. Some of them need 2 refills of energy to complete. Needless to say, that you will not only use all and any energy refills you have, but you will blow all your units to buy new ones.
2) The bonus side is that you get massive experience points, that lead you naturally to level up in game progression. Especially if you use experience boosts, the progress is sky-rocketting. Great thing, right? Yeah it is, and for each level up you get of course precious Mastery points. Hold that thought.
3) By fully exploring each Chapter of the 4, you also get additional Mastery points. Yeah! Now, of course since you do own these great Mastery points, you will want to invest them in the "good stuff" provided that you have your basics covered. So the "good stuff" being here Deep Wounds, Assasin, Recovery etc. Each level to unlock in these nice Masteries require (550 + 2x135) 820 units! 820 units for each level! The 5 levels of Deep Wounds for reference require 4,100 units just to unlock them all!
5) So, ultimatelly you WILL find yourself short of units, no matter if you play all the other parts of the game daily as I do (Arena, Alliance events, monthly EQ, you name it). And that will be short of LOTS of units.
So, the options here are that either you will take your time and do the exploration in months, having in the meantime un-invested Mastery points since you cannot unlcock them immediately (what a waste that is, right...?) or you WILL eventually be forced to buy units with actual cash.
Let me clear that this is NOT a complain post. I do realise and appreciate that Kabam is (naturally) in this to make money. And money they make. Nothing wrong with that. I am just pointing out the brilliant design of this game. That's all.
Thanks for reading this far. Feel free to discuss and / or offer any ideas.