I would try and cut out the rot and scarf in new timber, failing that, why not just buy some "Gitrot" and do the job with that, it will probably be cheaper and easier in the long run and you will know you have the right stuff! Use the epoxy later to fill any voids, after the gitrot has done its job.
do not mess about the only real answer is to get rid of the rot and replace with new timber. if its got into the ply the chances are that it has travelled between the layers far beyond what you see. whatever you do with epoxies and fillers meths etc will only put off the real job try not to do the job twice do it right first time.
I had suggested using that method for someone asking for a temporary repair.
I have used this technique mainly for saturating new plywood for all sorts of uses as it is a great deal cheaper than brand name products that do the same thing, and have used it to harden up small areas of rot in non structural situations.Rotten plywood is best cut out and replaced. I mixed the epoxy first and then added the meths. I suggest that anyone wishing to use this method should try it first on some scrap timber to satisfy themselves as to its effectiveness.
I have a relative who is an epoxy chemist and asked his opinion on my doing this. His reply was that it would be ok as long as one didnt try to glass over it before the meths had evaporated off or I would run the risk of getting bubbles under the glass sheathing.
The Git Rot product seems to do a great job for those who can get it at a reasonable price.
In your situation Fatipa the hardest thing would be getting it really dry and free of salt. I can understand why with a 4 year old boat someone may wish to stick to brand name products.