Unless I'm reading your original post wrong, you're trying to make a 3.5 inch wide dado or grove the length of the 4x4. A 4x4 is only slightly thicker than 3.5 inches. So I think you might mean you are cutting a 1.5 inch wide groove across the 4x4. Is that correct? If it goes the full width of the 4x4, then I'd buy a hand saw, and probably a Japanese pull saw, mark the depth of 1.5 inches on both sides of the 4x4, and cut the outside edges of the dado first (just shy of the mark), then make many cuts with an ordinary saw, or even a circular saw, and then chipping out the remaining pieces with a very chisel. Then use a chisel to pare the bottom flat all the way across. Don't use a mallet for the final flattening. Wiggle the chisel back and forth and take very light bites. Use a straight edge or ruler to check for flat, and for the exact depth of cut.
This might be the time to add a couple of tools, such as a combination square and GOOD chisel set. These will last your lifetime, travel easily and you'll find many uses for them. A speed square is cheap and can guide a hand or circular saw to make square cuts. You can get a lot of projects done with a decent circular saw and a straight edge. Mine is a Makita, but there are many other brands, including AC or battery powered ($$$$$).
The more I do, the less I accomplish.
Last edited by DesertRatTom; 11-12-2018 at 12:44 PM.