I had like 6-8 weeks before, no issues... and that is on my old R33 which at that time had a 10 yr old battery original from japan.
if you're worried, yeah just disconnect the battery. I would close all window, pop bonnet, close all doors, lock it, and disconnect the battery, and leave the boot lid down so you can lift it back up without having to worry about the window glass gets clipped over the top by opening the door without power.
it's better than leaving the window open for spiders, etc to crawl into your car while you're away 4 weeks. you'd be amazed how fast they can colonise your interior in a day or 2, let alone 4 weeks.