I presume this is intl to intl? And this is during the day? For most people, I'd probably advise to either 1).enter Jpn, stay at HND and check out the landside shops/restaurants; or 2).stay airside and use the lounge if you have access.

Me personally, I'd head to Kawasaki (Keikyu Kawasaki) which is ~20min by Keikyu train and I'd probably have ~90min to spend there. Kawasaki is probably the most worthwhile area within 20minute radius of HND. A little longer train ride will open up a lot more options out towards Tokyo, but it'll start to get tight with a 4hr layover. And I always have tons of stuff to buy in Jpn, I know Kawasaki inside out and know exactly where things are, so I can get a lot done in 90min. But that probably won't be the case with you. I think the airside facilities at HND have enough things going on that a couple hours will fly by fast.