Go have a look at the thing, have a drive of it, have a poke around, firstly have a look round the forum for general advice on buying one.
Service stamps are an indication only of the general vehicle condition, I've had my Disco TD5 for 8 years now and during that time hasn't had one main dealer stamp, reason being I wanted the servicing done properly, not just the oil topped up and the thing valeted and also for myself several hundred pounds lighter.
I give the Disco once a year to a semi retired mechanic, who had his own business, I supply all the service parts required and the disco is checked, serviced properly and also MOT'd and comes back with the service book stamped with his old garage stamp. The vehicle is now 12 years old.
Think the condition of the vehicle is more important that the amount of stamps a book contains. However if the one you are looking at has 4 stamps would that not equate to 60000 miles, a fifth one being on the 72000 mark, meaning it would be due shortly?
That said, on an 8 year old vehicle that's only been serviced 3 times, says a lot about how well its been looked after but still worth a look and a poke, might be a reason for a little haggling to reduce the price