Damn I didnt even know there are two more factory slots. But considering I am sitting at barely 732 cores not sure I unlock them, seeing as I simply dont have enough resources to keep mashing out new tanks anyway (cant access the game from work, the whole Nutaku page is filtered as Pornography and thus blocked by our filters, applies weirdly to the non adult Page as well.).

So far I got only one 4* tank from crafting, and one from the first Gacha you get for free. Most of my team is composed of 3* with bunch of 2* as swap in for some missions I use for grinding (there is one very easy to do - the map is clear of any obstacles and the enemy spams only light, medium and a few destro tanks so its very easy to blob up and overwhelm them if you start with heavy and destro.) - its the last one beforebefore you get to your ... what is it... 3rd Mine, at the south, under the portal.
Costs 107 oil and gives good amount of gold, and drops quite a lot of 3/2* and occasionally a 1* tanks.