I agree with Ryosuke. There is a world of difference between owning a good 4* team versus even having just one 6*.
When I started out I was excited at every single panel event because it meant free 4* units for me to use for my teams. But once I got my first 6*, it was like all my previous cards were worthless.
Another way to look at it, plenty of 4* players who get their first 6* will dump their allies and look for only 6* allies. Not as many 6* allies however will jump ship if they get a 7* and look for a completely new set of allies. So long as you've got a single 6*, you're cool and most allies will keep you no matter what. The 6* "fraternity" is very marked in this game, and very strong too. If you've only got 4*s and an occasional 5*, you don't rate and you'll often find allies leaving you if they get a 6*. Even having only one 6* will get you stronger allies which helps you for all sorts of events, which helps you to get more 6*s.
Just from playing and accumulating all sorts of free tickets, I've got over 500 4* units in my inbox that I don't need. If you simply log in each day, you'll eventually get more 4* units than you know what to do with.
Focus on getting that first 6*.