The problem here is that the term cob is used to describe so many different types of horse. Any well-built 15hh-ish horse, whether native, hairy, traditional, hogged or just fat is generally called a cob, but they are not all the same. There was a well known cob on the show circuit who is the happy accident between a Heinz 57 and a Derby-winning racehorse - there is no way his characteristics can be lumped in with a Welsh D. Just being the same height and build does not mean they are all bolshy tanks! Nor are they all safe and sane. Just as with all horses, you get good ones, and you get bad ones. The good ones are generally so good though that people tend to really sing the praises of a good cob.
My own horse was advertised as a cob. He's not really, but he is hogged and came from a cob-dealer and doesn't fit into any other bracket so that is what people describe him as. He came over from Ireland as a 3yo already with experience of hunting (not saying that is right, but it is what they do.) By 7 he had 4 full seasons under his belt, including a season whipping in, he had attended RC with an adult and PC with a teenager, been placed showing, XC and eventing. He is 9 now, has won the last 2 competitions he has been out to (I don't get out to many - too busy taking my kids and their ponies out!) He is well-known locally as a schoolmaster. I certainly paid more than £4k for him and I would also certainly get more than that for him now.
£4k is not an unreasonable price for a horse that theoretically is just beginning the best years of their life - by 7 they ought to have seen a bit of the world and have developed their personality as far as it is going to go. There will still be education needed, but by this age the basics should be in place and there should be plenty to build on if you want it, but enough in place if you just want to enjoy more gentle riding pursuits.
My friend is looking for a new hunter this year, and there don't seem to be many decent horses out there for less than £5k. You'll always get someone who says they never pay more than 20p and a packet of crisps for a horse, but they are either lucky or very, very knowledgable. For the most part a horse that someone has spent £1000s of pounds and years of time getting to the point that it is a safe, sensible, enjoyable animal to have around is not going to be going for peanuts. And don't believe it when people say this is a cheaper time of year. It is cheaper for outgrown kids ponies and show horses and eventers at the end of the outdoor competition season. But if you are looking for a good hunter for the season, or are a dressage rider or show jumper who is looking to buy next season's prospect to get to grips with over the winter by getting out to few indoor winter competitions, now is as good a time as any to buy. You 15.1hh cob could potentially be a decent hunting horse - it is a popular size for mother/daughter shares, older riders and riders accompanying children on ponies (i.e. people like me!)
If it were me I would have a vetting done and subject to that pay the £4k. The greatest cost in having a horse is the keeping of it, not the purchase price. If you buy the right one, the extra 500 quid will soon be forgotten.