A bit more information might narrow it down...
What is the diet of these two? What is the pattern of hair loss (generally or patches in certain places?) What is the skin like (pinkish, hot, crusty etc?). Is it bothering them - are they scratching? Are they actually losing fur - were they fully furred before, or are they being slow in growing their fur in the first place?
The 2 things I'd suspect most in hamsters losing fur this young would be mites (especially if they're runts more likely to be immunocompromised) or dietary deficiency. I'd boost their nutrition (offer porridge/baby cereal, small pieces of ham, mild cheese, boiled/scrambled egg, tofu, cooked beans etc) and add a good vitamin supplement to it. That may be enough by itself but if possible some ivermectin treatment from the vet may help things along if it is mites.
Being me I'd also add a pro/prebiotic supplement to their water (I use probiotic with anything remotely ill!) as they're so young, something is obviously going on with them and if they came down with stress-related diarrhoea at this age it could be very serious. Probiotic may help prevent that.
By 4 weeks these little ones should also be sexed and separated - I'm sure you have done
but just checking that these two are the same sex if they're still living together.
Em x