how far into the fields did you go? I'm guessing you'll be able to see signs of a rabbit warren round the edge of the field somewhere - probably over the other side away from the houses though as rabbits aren't likely to build a warren right next to a housing estate I don't think. If there are woods over the other side of the field then it may be there - something like that will attract them to build because of the tree roots.
If it's only 4wks old then it's a baby and needs to be with the warren, although the longer it's gone, the more likely they are to kill it when it returns anyway - knowing bunnies