I’m with @trpnhntr. Just go race and don’t worry about being dropped. You need to learn how to navigate the field and learn to be efficient.
Having said all that - for flat crits in NorCal, I find it’s less about w/kg and you should be able to average 180-210 watts for 45-50 minutes to hang in the pack
I raced my first Cat 5 critical at 4 w/kg and finished mid-pack. Also felt shelled.
It’s an interesting question, but a hard one to answer. My race was fairly technical, so there were a lot of hard start out of corners. My repeatability was low - and that has way more to do with you aerobic conditioning than your anaerobic capacity (counterintuitively).
I agree with just racing no matter what. My first race was a 30 miler and I was still very much a “recreational” weekend rider. I came in dead last, and actually almost didn’t physically finish and it was still one of the funnest races ive done. Winning a race is not necessarily the fun part. Lining up to a race is a victory in it’s own.
I did my first races last summer: flat 3 corner crit. I was 4.3W/kg and got flushed out the back after 15 minutes of utter hell. The race was about 30 riders going through and off trying to not get caught by the really fast group that would follow 5 minutes behind, so it was non stop intense. As the weeks went on I got better at efficient riding and practiced a spiky repeat sprint power profile so i could hold on for the full race. It would be between 290 and 320W NP for an hour’s race.
So just a decent FTP is only half the story. This year i hope to apply everything I’ve learned from Amber, Pete and the team about how to race…
I already race, I am just curious what sort of fitness is needed for entry-level (Cat 5/4/D/C) races in places that actually have them.
If you consider fitness as the ability to perform an activity and define “activity” dependent on the type of race, then you’ll be better equipped to decide on what training you should do.
For example:
Criterium racing often requires a short period of work above your threshold. Repeated many, many times. If you can’t repeat those efforts you’ll eventually get dropped and not enjoy the race.
So, in our example, the “fitness” is “being able to repeat short intense efforts for the duration of the race”. With that in mind you can go and choose a training plan that targets that fitness requirement.
No need for numbers because honestly, it doesn’t really matter…unless it’s a TT.
I’d echo a couple of the comments on here about not waiting until you think you are ready, instead just have a go and see how you get on.
Last year was my first experience of any sort of bike racing. I entered 6 criterium races on different sorts of circuits across the season from April to August.
First race was a shock - got lapped 15 minutes in (very short course 4-corner car park crit with a sub 1 minute average lap time). I was unprepared for the immediate pace.
Second race on same course, lapped again but this time over 30 minutes in before that happened.
Third time a charm - finished 12th in a field of 22 on the same course - felt like what I learnt in the preceding 2 races better prepared me to hang in there and also pace myself better
Then had some fun at a town centre multi-corner technically hard crit - came about 14th IIRC out of a field of around 30.
This is all UK cat-4 and was mixed age groups - mostly (over 90% by my observation) sub 25 year old participants. I was 46 at the time. All
the races were about 45 to 50 minutes in length.
Came 5th in an oval track race (only 8 entrants so no points picked up) and ended the season with a race at a motorcircuit - came 12th in a field of 26 or so people - again mostly youngsters - my ftp had gone up a bit - maybe 3.3 / 3.4 W/kg by then. I was maybe 2 bike lengths from getting into the top 10. Maybe another 4 bike lengths plus have seen a top 5 position - it was pretty close at the end and that helped my self confidence quite a bit
For info to answer the OP question, was around 3.2w/Kg for the first race in April
So I picked up big fat zero points and didn’t even get a genuine top 10 finish, but REALLY had an awful lot of fun and learnt so much from the experience, in terms of things like:
Regardless of the results it was really good fun and I am determined to go again this year coming and at least pick up some points by getting into the top-10 along the way.
Good luck 
I’ve raced at 3ish and been dropped every time. I think my reasons for being dropped include bad strategy (picking wheels to follow), not pacing well, and a horrendous CdA/not-aerodynamic position on the bike. Probably others too.
It’s been awhile since I’ve raced, but I’ve done a number of CAT5 road races and crits and every time, the key thing was weathering the 32-34 mph out of the gate until people got real. A couple of races got pretty easy after that… where a 2.5-3.0 w/kg would’ve been plenty to be able to hang, then make a move at the end.
My thought is that your w/kg is only a part of the equation. On the other hand, if you’re enjoying anything you aren’t racing hard enough! 
I’m just over 4.0 w/kg racing cat 1 mtb and it ALWAYS hurts!