Well, tractordata.com doesn't list the speeds for the early 4 cylinder 4000 series. I'm thinking that it's gonna be similar to the 800 series, which shows first gear at 2.3 MPH. I've run a 4' tiller behind an 8n, which is way too fast at 3.7 and doesn't have enough HP. I'd think the 4000 would be worth a shot with the 6'. Worse comes to worse, you'll have to make multiple passes. Also, as someone suggested, maybe a disc and drag harrow would be better. Certainly cheaper.
Personally, if I was buying a tractor just for food plots, a new Kubota wouldn't be on the list. Maybe a Deere 3020, or something like that. There's even a used 5040 Kubota with a cab on our local Craigslist. 700 hours and 17,000. 2wd and no loader. That'd put in some food plots for less than any new Kubota. To me, the appeal of buying new is getting 4wd and a loader, but you don't need that to run a tiller.