I've tried gluing, using larger screws, and got a cover for the flaking plastic, but there's just no hope for the original HE-4XX headband. I put aside my 4XX headphones, but I couldn't bear to throw them out -- they just needed another headband, and most of all, I liked their sound.
Hifiman has a couple of replacement options, but they aren't cheap, and they don't specifically mention the 4XX, so I have been hesitant to purchase. I've seen some of the writeups on modifications to swap headbands, and I'd hate to spend a lot of money to end up with something that still didn't work in the end. And I surely didn't want to buy another set of Hifiman headphones just to swap out parts.
So, what's changed? Someone on Amazon is now selling a generic headband that looked like it would fit, and while not cheap, it's cheaper than the Hifiman options.
Basically, the hardest part was finding a tool to unscrew the earpieces from the yokes. When none of the hex tips I had would fit the screw, I ordered a set that included a couple of smaller sizes. (I think 1.5mm fit.) I simply re-used the existing screws and rubber washers, reassembling the cups onto the new headband. Pretty easy. No drilling.
Here, you can see they look OK. Metal band on top, leather band on the inside. Not fancy but also not flaking all over the place.
If interested, look for the "Voarmaks" brand on Amazon. I wanted to post this writeup in case anyone else is looking for options.