The following will force Embree to use SSE4.2 only:
./pathtracer --rtcore isa=sse4.2 -c cornell_box.ecs
The following will force 4-wide BVH for triangles but still use modern ISA if possible:
./pathtracer --rtcore accel=bvh4.triangle4
The following will give you some output of algorithms chosen to verify that everything worked as expected:
./pathtracer --rtcore accel=bvh4.triangle4 --verbose 2
No leaf size cannot get changes that easily. Also not that Embree always packs 4 triangles inside a leaf anyway (structure of array layout), thus going below 4 makes not much sense anyway.