I think you might want to consider planning for a race in the fall rather than in the spring. With illness being much more prevalent in the winter, this would at least give you a chance to have a clear runway with health up to your race date.
I don’t really see how you can mitigate the effects of missing 3 to 4 weeks of training four weeks out from your race I don’t think there’s any special training that could offset the loss to aerobic fitness that occurs with that long of a layoff.
I hope this helps.
Scott