My bitch was scanned at 28 days, she is in whelp with four puppies but one of them is smaller than the others. Should I be worried? Do you think it will catch up? Has anyone had a similar problem? Thank you.

I would not be worried, puppys are not all the same size when they are born, and its often the small one that ends up overtaking the others, im sure it will be ok ,they have a long time to go yet .

In my last litter, which was of 4, one was much smaller than the rest and one was absolutely huge, the other two in the middle. Now when they are adult all four are the same size.

Not sure about on the scan but my bitch was the smallest in the litter. She is not fully grown and she is now bigger than her mother and I can't see any difference between her and the one the breeder kept size wise and I think their other sister is about the same from what I recall...
One of my breeders years ago said the smallest often turns out to be the biggest when fully grown!!

I assume the one is much smaller than the others if it is noticeable in the 28 day scan?
It is possible the pup is not getting nutrients it needs (placenta not working properly).
Worrying about it will not help at all so I think you will just have to wait and see how many will be born and be prepared to possibly have a very small puppy that might need extra care.
Or you might just have 4 similarly sized pups as the little one could catch up even before being born :-)

Very rare for a scan to be even close on numbers, and as someone else said worrying now is pointless, and of course knowing numbers inside is pointless as they have to reach term (bitches can absorb up to 6 weeks) and be born safely.

My last litter the bitch carried 11, but we end up with 7 live pups, and 3 of the four stillborns were the bitches i wanted.

Chances are you'll get a healthy litter and quite probably more than the four seen if we are talking medium/large size breed.

I had a small puppy in my litter and she still remains smaller than her siblings, I thought she would catch up but actually she was always tiny, I groom her sister who is a good inch taller than her at least.

Both the scans I had done (13 years apart but by the same scanner) were spot on for numbers.

That's fairly unusual, unless the litters were smallish.  In the USA a lot of people have the bitch x-rayed for puppy numbers a week before the due date. 

The litter my friends imported youngster was from said 54 pups on x-ray, yet she had 6, so even that is not certain.  I had questioned the point of x-raying, and it is done to know how many pups to be sure the bitch is finished.  In that case had she had five and the other been retained, they would have thought she was done.

> was from said 54 pups on x-ray, yet she had 6,

54 pups??!!

54 pups??!! eek

LOL bet that's a digit wandering to surprising effect.

Yes, how did I miss that it was FIVE.