Yep 4th cat is where you start. I’d say just turn up to some races and give it a go.
Are you a member of a road club? If not i’d recommend joining one. You learn lots off other riders and racers and can practice drills, riding in a group, drafting and pace lining.
The thing about road racing you realise it’s not all about fitness. I got caught out many times with this.. bad tactics, bad positioning, not reading the race very well..going too hard when it was my turn to pull.
But I learnt lots from it.
If you can afford it have a fitness test to see where you’re at. I did this last november and did 5 months of religious training (av 8 hrs a week)