Shimano is conservative sometimes... But I recently setup a GRX 812 on a 11-46 per a request. The GRX 812 is spec’d By Shimano for 11-42 max, but some have stretched it to 11-46. The issue we ran into is the B screw is so far ‘in’ to get it to shift the 46, it shifts poorly in the lower 2 cogs. He was willing to accept 2 sloppy gears to get a 46T bailout. Personally I’m doing just the 42T (Shimano recommend max) as I can’t deal with sloppy shifting.
Please post up your GRX400 findings after.