Don't let yourself be put off by the 4th fleet type. You can still run a very profitable airline with 5 fleet types. Problem is you need a huge amount of each fleet to justify the added expanses. I borrowed money from the bank to start 767 operations with 23 aircraft in a fifth fleet group. It could have worked well if competition didn't drop their prices under a point I was no longer able to sustain the debts. So after 3 years I had a clear bank balance, borrowed the amount needed to terminate all the leases and from one day to another the entire 676 operation was seized, including some profitable routes. After that I spotted a jump in revenue after taxes. So if you do a fifth type, make sure you get money from it otherwise be prepared and able to terminate an entire business.