That is a big thing. Yes, go with that. No worries.
Reducer? No. The pump is about 3.75 to 3.9 od. You will cut narrow wedges from the top of the sewer pipe to make it easy for the worm gear clamps to tighten the pipe around the pump part ("wet end") of the pump. You can use Scotch 66 tape from HD, tho the pros use a wider tape sold for well work. Were you thinking of adding a 3 inch diameter tail? No need for that. Just let that tail part be 4 inch. The large diameter of the pipe will make the 9 gpm water velocity low. Sand will not get sucked upward.
The Sub-K adds a centrifugal aspect to the process. Your sleeve will not do that, so it would not be as effective against the finest sand.