Hi Maria,
the general rule is to separate once blood has been drawn.
I think it is definately a good idea to remove the injured one from the cage - he might get picked on even more, as he sounds like he has been injured quite badly - especially as he is losing weight.
You should house him separately, and once you get back from the vet, as long as he has been given the all clear, try to do as much as possible to aid his recovery - not sure what you are already feeding them, but try him with mealworms (dried or live) and some millet spray too - both can be quite fattening, and have lots of protein too, so feeding him with them will help his recovery - you can feed the others the same too - but only in moderation.
Once he is completely healthy again, you could try introducing him to the least dominant hamster of the remaining three - but, it might not work out and then you could have two on their own, as you might not be able to re introduce him back to the other two - it is completely up to you wether you want to try introducing him again - you might end up with all 4 in separate cages in a few months!! - it might happen anyway.
Also, with the ones that you do have living together, make sure they have one each of everything in the cage - wheel, tubes, water dispenser, sleeping areas etc. - and it is best not to use food bowls - just scatter the food all over the cage floor.
Good Luck at the vets.
Keep us posted
Tammy x