I took my toy bitch on exactly first day of the 4th week after she had mated and vet said she saw no puppies can she be wrong? My pup having signs of pregnancy like eating a lot and sleeping x

It is possible yes and 28 days is sometimes a bit too early. But at 4 weeks pregnant I'd expect to see sickness and the bitch going off her food, not eating more.

Yes 

Yes she has defo went off her food the last few days defo not eating the way she was before x

Just to add not all bitches change much after being mated.   Some of mine would do the classic - bit of morning vomiting.  They rarely went off food and although some would be noticeably more careful with themselves early on, most didn't until their increased girth slowed them down.  Bitches don't read text books, and even another pregnancy in the same bitch, can produce different behaviour.   If you know your bitch well, you may notice a slight thickening of the flanks by around 4 weeks.   And obviously you should see puppies moving around by the 7th week.   You are playing a waiting game - be patient and plan for a litter.

The vet might be right but I would be looking for someone more experienced to scan her.