In my honest opinion, while I don't necessarily agree with jumping a horse that much at such a young age, if she is built well, in good condition, and has a solid, extensive training on the ground and in dressage she would do fine at one or two of these competitions as a four year old as long as she has adequate leg protection as well. I know several trainers that specialize mostly in the YEH series (young event horse) in the United States and the 4-5 year olds they compete at similar events to the one you described have gone on to be completely sound as they move up the levels. I personally know two horses that did YEH series as 4 and 5 year olds and they're 8-9 now and sucessfully competing at preliminary and training levels (not sure what the equivalent of that is in IRE, sorry) without any soundness issues.
If you're set on this I would suggest trying to find a trainer at least sort of near you, or even one you could email, that specializes or has experience in taking horses to these young horse events and picking their brain about the training you should be doing and at what ages you should be starting your mare on certain things.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."