I swapped out the stock headband of my 4xx for an HE560 headband and it's great. Improved comfort and fit which I feel helps created a better seal on my head and thus improved sound.
Someone had posted this mod a couple of weeks ago and I thought i'd give it a try. I ordered the HiFiMan Headband for V560V2 from Adorama for $40. It's an easy install. You have to stretch the yokes a bit, but it's not difficult. I have the 4xx with the 2.5mm cable connects. The size of the hex screws that hold the headphones to the yoke are 1/16. Make sure you have the right size hex tool the screws are easy to strip.
Not only does the headband look better it's way more comfortable. The 4xx don't feel as heavy on my head. The clamp is tighter than the stock headband, but not bad. I expect it'll loosen up a bit over time. I wore the 4xx560 for about 90 minutes straight today with no fatigue or hotspots on my head. I recommend this upgrade!