I used to have the same problem on my old 15.5m but mainly in lighter iffy wind.
Spend around $30-50 to get a 5th line attachment sewn in and around $30-50 for a new 5th line (same dyneeda or whatever you existing lines are) and you will never regret it. I did and the old beast is much more fun. Relaunching is a piece of proverbial, the bonus is the whole set-up is safer too.

Make sure you get a yoke/leader line on the kites attachment point with several knots, to play with getting the right length. When flying, you want the right length to be just a little slack on an old 4 line kite - not taught so as to pull the LE forward/depower. A ring at the bar end will act as a stopper/leash attachment.
Ava search on SB for a converting to 5th line topic etc.