Quote Originally Posted by Unregistered View Post
I opened up the fifth factory slot the other day. Not a single regret, because now my base is completely finished. There's always the option to spend cores in the store later if the events don't suck them all up, and all the while I'll still be cranking out new tanks in the factory for basically free...in fact, I just put two 4*s into production while I was composing this post.

The store is a fool's bet for FTPers, even at the 'generous' 3% feature rate.
I can't recommend the 5th factory slot simply because 2700 cores is such a high cost for so little reward. But the 4th slot is a must, be aware that it will likely take you 600-700 builds to get a 5* out of the factory on average but with all the 4* you will get that 4th slot is well worth the investment.