Here's what I do, it's a little complex but it works!
First off I have an app on my phone called Private Secure Calculator. This hides a secret photo cache behind what looks like a calculator app. While the app IS a functioning calculator, if you put in the right combination it unlocks the secret photo cache.
Ok, so first I lock my chastity device on then place the key inside a Masterlock key box. Without looking I jumble up the combination and close the box. Now the tricky part. Without looking at the combination I use the Private Secure Calculator app to take a picture of the combination. Basically I make sure the words MasterLock are in focus to make sure the combination is readable and in frame. If I accidently see the combination I just start over.
Once I have a good picture I scramble the combination on the MasterLock. Cool now it's locked and the only place it's recorded is on my secret Calculator app. Now I use ChastiKey to generate an eight digit combination. Before I hit accept I use this eight digit combo as the new unlock code for the Private Secure Calculator app. I close out of the Calculator app which locks it, switch back to ChastiKey and hit accept.
Eight digits are MUCH harder to remember than four. Hell you could make the code 12 digits long.
If you STILL are having a hard time forgetting the combo, congrats you have a great memory! That said you could also play a few rounds of this Sequence memory game. I find all of this works really well.
https://www.improvememory.org/brain-games/memory-games/sequence/Of course any procedure with this many steps has plenty of ways to screw up! I'd practice several times before you lock anything vital up, and you may want to figure out some sort of emergency release system. Hope that helps!