I once missed nearly two weeks of decent training during build up for VLM while I went to Sydney. I managed 22 miles in the first week with a 10M long run and 39 the second week with a 14M long run (admittedly, both done on home soil before and after said holiday) The 14M was tough but I was pretty jetlagged having only got home the night before. I then had six weeks until marathon day, so it really wasn't good timing.
The runs I did involved alot of very early mornings after rather late nights! I did really enjoy the runs as the place was beautiful so early on a morning.
The marathon didn't quite go to plan, although I was probably aiming for a quicker time than I was capable of at the time with or without the holiday. It was my seventh marathon, I still managed a four minute PB!
Depends on how much you really want it. One run a week for four weeks is nowhere near enough to be honest. Pack your trainers. Get up earlier. Don't underestimate the marathon or it'll eat you for breakfast, believe me. Either that, or defer your place until next year. They're so much more enjoyable and rewarding when you've put the miles in and have a fantastic run.