Here are potential issues that may or may not arise from such solution that I see.

1. Geometry changes. As it is always been, control that the head tube angle of a bike changes in a way that does not worsens its handling. I, for one, use 27.5+ wheels and tires with a fat bike Surly fork originally shipped for 26" wheels with 4.8" tires; but the vendor in this particular case states compatibility with 26", 27.5" and 29" wheels, and so far it was so good.

2. Clearance issues. Yes, a 27.5+ tire alone would fit; but would all the mud and snow sticking to the tire fit? I've had situations in my life on some bikes when either rear or front wheel got clogged with a mixture of clay, grass, sand and dirt, or a sticky snow.

3. Tire width variation outside of vendor specs. The actual installed width of a tire depends on the width of the rim used, and it could be wider or narrower than specified. So your 4" can become 3.8" or 4.2" in reality.

For other, a trail fork should be as good as a fatbike fork, or even better. For example, RockShox Bluto is considered to be "noodly" because its stanchions are just 32 mm and seems to be derived from a cross country fork. So a narrower fork with 34 mm stanchions could be stiffer.