Imo , I think the turning pro element of the original post is beyond the OP , however racing is not.
I have no figures on the standards of 4th cats, but I imagine it will take a bit of work to get competitive, but don't let that stop you giving it a go.
I can give you a bit of experience based on my Athletic career though.
At the age of 25 I started running, within 6 moths I felt I was capable of handling some road races.
Depending on the race, I was in the top half (bigger mass participation 10ks) or back 10% (local Athletic club organised events) but I still enjoyed it and it spurred me on to join the local club and train harder in the road running group.
Ended up doing the national xc champs , primarily because not enough club members better than me wanted to go all the way to Leeds, got lapped by Eamonn Martin et al and finished about 900th out of 1050.
5 years later, I ran the Essex 20 mile champs and was so far off the back they didn't record my time, but I kept it up, I enjoyed the occasion, but I was getting nowhere. I had a word with my coach a few months later, who said that, in all honesty, I was not cut out for distance and, given my various training performances, I ought to stick to what I was better at and enjoyed more, shorter faster stuff.
2 months and some really intense track training I made the clubs 3rd team at 400m.
Sure it was only Southern League div 7 , but I had my Mojo back.
I carried on, competing where and when I could. Div 7 , I was competitive, the day I first won a race was great, (Bournemouth July 92 I was 34).
County Championships were an experience, competing with internationals and watching them disappear into the distance..
I ended up doing the World vets in 99
Got a GB vest and all the kit, (had to buy it) , you just entered, it was not a selection thing , there were over 30 brits in the M40 100m alone, and I got thrashed, but it was one hell of an experience.
What this long winded diatribe is trying to say is, have reasonable goals, and realistic ideas of your actual standards, work a hard as you can to first reach your ideals then look to gradually improve them
Don't let your standard stop you competing in the better quality events , but expect a stuffing and used the quality to motivate you to pbs.
Train with a group , hard work and dedication will gain you the respect of your peers regardless of your actual standard.