According to this link on Singletrack you need the X12 end cap. Hope Boost hub and Shimano 12 speed - measurement needed
From that link, "That end cap looks too short. I have a Pro4 and had an xd driver on it initially. When I went to shimano 12 speed I bought the correct driver and what was listed as being the correct end cap, but it was too short. I rang Hope and asked what it should be and I'm adamant that what I'd ordered from CRC should have been correct. I think CRC had a listing error for a while that is now correct. I found what Hope said I needed in stock at SJS Cycles - the detail is below:
1 Hope Pro 4 MicroSpline Drive Side Spacer - X12 - Silver - HUB1016-03S Product code: 52571"
A forum community dedicated to Mountain Bike owners and enthusiasts. Come join the discussion about bike parts, components, deals, performance, modifications, classifieds, trails, troubleshooting, maintenance, and more!