If you're even vaguely a history guy, I'd bite the bullet early-on and accept that you'll go to Little Big Horn even if it makes little logistical sense. You're going to go to Mount Rushmore for the same reason. You've not likely to ever return to this part of Big Sky Country and you'll almost certainly kick yourself farther down the road if you were relatively so close to the Little Big Horn, but didn't bother to visit for fear of a 4-hour drive.
IMHO, it's worth the trip if you can combine it with another desirable destination (in your case, Devil's Tower). Your best bet in this regard would be to take a 2-day, 1-night side trip from the Black Hills to Billings (Bozeman is actually much cooler, but comes with a fairly significant additional drive). On the way from the Black Hills to Billings hit Devil's Tower first (it's closer, so you'll arrive earlier than you would if you started with the Battlefield), then travel to Billings for an overnight. The next day, go to the Little Big Horn early, then return to the Black Hills (you might even be able to do a walking tour of Deadwood, etc. and grab dinner before heading back to home base). In this regard, you asked if there was anything else of interest in northeastern Wyoming. In my honest opinion, no, especially given the time that you have available. You're efforts would be better spend exploring the Black Hills and its environs.
For what it's worth the Little Big Horn Battlefield is relatively limited in scope. Even a comprehensive tour won't last more than 2-3 hours. It's not like visiting Gettysburg or Antietam or Shiloh or even Yorktown or Saratoga. Probably no battle on American soil has been studied in more detail. But, that's because the relatively limited number of engaged US calvary makes it easier to do a deep dive. As others have indicated, there are markers identifying the "exact" spots where each soldier perished during the "Last Stand." However, Custer, himself, was reinterred at West Point.
You can do a mini-bus tour of the entire battlefield, generally guided by students enrolled in a history program at the local community college on the Crow Reservation and there's a solid, but not amazing museum. Unlike Fort Ticonderoga, The Alamo, Fort Sumpter, etc., there is no physical centerpiece to attract visitors and, frankly, the number of dead is quite limited when compared to virtually any other major battle conducted on the continent. However, the Little Big Horn is also a national cemetery--like Arlington--with numerous veterans interred by virtue of service in other wars. It's iconic and worth the drive, provided it's coupled with other interests.
For me, there were three interesting take-aways with the Little Big Horn. First, as the battlefield is literally in the center of a Crow Reservation, most staff, specifically including tour guides, are Crow and very much explain the battle from the perspective of their ancestors--who pleaded with the US Government to remove their traditional enemies, the Sioux and their allies, from their reserved lands. The Crow largely supported Custer's efforts, served as Army scouts, etc. As such, the battle is NOT explained through an oversimplified "Custer deserved what got" prism, I was impressed by the nuance.
Second, while still depressing, as viewed from afar, the Crow Reservation seems to be among the most successful reservations that I have ever had the occasion to visit--much more vibrant than the nearby Cheyenne and Blackfeet and, light years beyond, the inexcusably impoverished Lakota Sioux at Pine Ridge in South Dakota. The extreme poverty at Pine Ridge will make you and any other rock-ribbed Republican cry. The US Government stole their land and gave them shit in return. This isn't even a debatable point.
Third, unless you really know what you're doing, DO NOT walk into the tall greasy grass that surrounds the battlefield and covers the entire valley below where the Sioux and their allies camped in the days before the battle. No joke, there are prairie rattlesnakes everywhere. Being from upstate NY, while on hikes, I look-out for venomous snakes (timber rattlers) in rock outcroppings, under logs and on paths covered with leaves. In the plains, their prairie cousins simply hangout among the tail grasses and seldom give much warning before a strike. They're simply impossible to see. I was actually bitten while attempting to quickly take a look at a coolie. One fang completely missed while the second drew blood in a graze without releasing venom. Thereafter, I encountered, and successfully avoided, 2 other rattlers, just to walk 50-feet back to manicured grass.
Again, this is an iconic American vacation. Enjoy every last minute!
