3/4 is a joint 3rd and 4th Category licence holders race - I'm a 4th Cat so bottom of the pile.
When you buy a racing licence you are start as a 4th Cat, finish in the top 10 in races to pick up points, when you have enough points you move up to 3rd Cat. If you don't have a licence you have to buy a day licence.
I gather the course is a box with 2 long sides which are a hill, so you have to keep going up hill, and some of these buggers do it at the same speed as the flat. 0.87km per lap and the race is 30 minutes + 2 laps.
To be honest from my experience at Palace the 3/4 races aint easy and I wouldn't enter this one (which according to Paul T might be harder) as my first race unless you are seriously masochistic.
Your best bet may be to wait until later in the year and debut in the ACC Road Race. A members only affair. Friendly atmos, you won't need a licence, and you can find out if you like it. That's what I did last year, enjoyed it so got a licence and having a go in a few races this year.
There are 4th cat only races which would be easier than a 3/4 but they're at Hillingdon on a Tuesday evening which is a bit of a faff for me, hence my kickings at Palace in the 3/4s. I'm not in it for the points (just as well really) so it doesn't bother me.
All that aside if you want to give it a go, what have you got to lose? I won't turn down the company. Might be worth checking with Paul T first for his opinion and how much it would cost to enter and get a day licence which are normally quite pricey.