
I never had a pregnant bitch go off her food until her size meant she had problems eating a full meal. At that point I'd cut back on the 'bulk' and feed a series of smaller meals. I'd suggest she shouldn't be off her food this soon in the pregnancy. Pregnant bitches should remain on a good quality diet, the same as she had before being mated, until again, her size dictates a slight adjustment is needed. And that would only be needed if she's carrying a big litter.
By around 4 weeks, owners might notice her being a bit more 'careful' of herself, around other companions (but not always!) and some thickening of the flanks but that's all. I always noticed the vulva tended to stay rather more enlarged/puffy than normal and maybe teats too with a successful mating, but again, not usually at only 4 weeks. Imagination!!
I'd not be happy to see a lack of appetite when only 4 weeks into a pregnancy.