not the best idea
a dbol only cycle isn't the best idea. to actually get much of anything from it you'd have to run it for probably 6-8 wks. a lot of what you will gain will be water weight which will come off once you stop taking the dbol. you CAn gain some mass on dbol alone, but you will be disappointed by seeing how large you get on it and then as the water leaves you see how small you get... small still being larger than where you started though.
running it for 4 wks by itself doesnt' seem very smart as you'll ahve to go into post cycle therapy to get your hormones back on track and it probably isnt' worth it for the gains you'll get.
if your going to do it anyways, DEFINITLY have some anti-Es on hand. nolva is good, but my advice would be to have at least a weeks worth of something stronger on hand so that if need be you can use that while you get some more. i'd say 30-40mg/day would probably be effective if your going to do it this way.