Your 4-Star 60 should knife edge very well, mine does, and I've had 2 of them. Mine is bashed into a "sprt of" Kawasaki Tony, WW II fighter. I did add a little extra to the top of the fin and rudder to get the scale shape, but not a significant amount. Here's a few rocommendations.

Check your rudder servo. Go to full control and push on the rudder. You may need a more powerful one to hold the rudder over.

You might try knife edging where your rudder servo is pulling instead of pushing. Pushrod bend can reduce the rudder throw. I moved mine to the rear.

Speed helps unless you have a good bit of side area. I am using a Magnum .91XLS with a 14-6. I had an OS .91FX in my first one.

Raise the nose about 20 degrees, release the elevator and roll into 90 degrees of bank.
Put in full rudder to start with. If your plane climbs, use less.
The rudder you use is opposite to the way you rolled in. Right roll-left rudder. Left roll-right rudder.

My 4-Star 60 does not roll, but has a very slight amount of pitch. It will hold knife edge all day.

Planes like trainers that have a high wing and loads of dihedral tend to roll out with rudder and are actually much harder to knife edge than a properly designed acro plane.

I covered flying knife edge in the R/C Report, May 2001 issue. You can contack them for a re-print at www.rcreport.ws. They advertise here on RCU.