I wouldn't neuter that early unless there's some serious risk and I cannot imagine there's any reason this would have to be done this early, in my experience of taking in rescue dogs that have had undescended testicles and of what other people with pups have gone through.
Generally vets say that if a testicle has not descended by 6 months or so, it probably won't, but some breeders who have a promising dog and don;t want to neuter unless necessary, will wait a few months longer.
Personally I think 4 months is way too early and I'd hold out for 6-9 months unless there is some reason for greater urgency. Problems from an undescended testicle don;t tend to develop that fast though! However they ARE a serious health risk over time and often become very painful for the dog internally, and have a high risk of becoming cancerous. This was the case with one of my rescue dogs who must have suffered for years with the pain and was thought to be aggressive because he didn't like to be touched. :x He eventually went on to a caring home but I did have the removed testicle biopsied and it was cancerous.
Note this vet recommends neutering for this AFTER 6 months of age:
http://blog.petmeds.com/ask-the-vet/undescended-testicles-in-puppies/#.UCBNVe3mGnw
However it may be that your vet has a good reason for suggesting this very early neuter and you should clarify if there is a reason. Perhaps s/he can feel the testicle is up in an area where it risks twisting around internal organs and be far more threatening to your puppy's health?
If this isn't the case, I'd wait a bit.
There's lots of debate around when to neuter. I did my boys at 9 months or so and they are standard height etc with fairly smooth, not curly, coats. Some like to wait til about age 1 but in males, this does carry a greater chance of some unwanted behaviours being permanent -- eg excessive marking, humping etc compared to neutering before the dog starts to establish these behaviours in adolescence. Given that many people give up dogs for such behaviour, I think a few months' earlier neutering is a very small tradeoff.