You can use 3/5 for that trill.
But...
It's well worth the effort to get a decent 4/5 trill. You will never know when you might need it and there will be no substitute! The problem has to do with the anatomy of the hand muscles and tendons -- over which you have no control -- and the nerves controlling them -- over which you have some control. The 4 and 5 muscles and tendons are very tightly linked; if you go talk to the average man or woman on the street, they simply can't control the two fingers separately!
The only solution is practice. Practice. Practice. You have to really think about controlling the two fingers separately at first, then with time, practice, and patience it will become more unconscious.