Regarding your 105 calipers versus the ZRACE XG4 4-piston calipers, your Shimano 105 have 22mm pistons, versus the 2 per side, 16mm pistons in the XG4 calipers. This results in about a 6% greater piston surface area for the XG4 calipers. (probably within the travel/modulation capability of the Shimano lever.)
I have had a lot of frustrating experiences with the basic 4-piston "E4" 16mm based MTB version of the XG4 caliper. I thought that this caliper would work fantastic because it is about a 90% copy of the Hope Tech E4 calipers. Not! For some yet TBD reason, it appears that the E4 rear caliper is very difficult to bleed. Looking at the XG4 caliper, it appears to have the same layout for the pistons and pads as the E4.
Therefore, I don't recommend the XG4 calipers.
If all you are looking for is new calipers, as stated already, I'de look at the Lewis calipers, or perhaps the new Wheeltop 2 piston calipers.
From another perspective, my bikepacking gear and my weight dials in around 210KGs. I have had numerous brake issues over the past several years. Last year, I started using larger rotors on the bikepacker. 180mm rear, 203mm front. I immediately noticed much better stopping power and less brake fade, and I still had about the same lever modulation. You might also experiment more with pad compounds.
I would recommend trying this before changing your calipers.
1. Purchase a set of caliper adapters and migrate to larger rotors.
2. Research brake pad compounds, and install new pads at the same time.
3. Bed in the new pads and test how they work.
Note: Migrating to 4-piston calipers that have only 6% greater surface area isn't a guarantee that the new calipers will stop any better than the existing ones.
- Unless you know you are overheating your current set-up on a regular basis, and have either cooked your pads or rotors, I'd stick with testing larger rotors and pads first.
