My thoughts based on my area. Might not work for everyone.
Common bermuda is not the best choice for a hayfield or pasture. Slow to spread and low productivity. There are seeded varieties like Cheyenne 2 that will out perform common. But those seeded varieties will not perform to the level of most of the hybrid sprigged varieties. I would be concerned with getting the seeds too deep with a drill. What I have seen here for a small acreage - Well prepared seedbed. Mix seed with sand. Broadcast seed/sand with an electric driven broadcaster or one of the old hand crank broadcasters. Can be done from the back of a truck or utility vehicle in low speed. Then run a cultipacker or roller over the soil to firm the soil for good contact between soil and seed. TRY to time it with rain and right time of year. Competition from weeds and crabgrass will be an issue. Unhulled seed can be broadcast very early and some will sprout later based on temperature and moisture. I think it is way too late this year for seed.
What I would do a small area like your 4 acres - find a friend or neighbor with a hayfield or pasture with a hybrid sprigged variety suited for the area. Just before or during a rain, hand pull long runners from the edge of the field or under the fence (where they have not been cut or grazed). Longer and coarser the better. Load them in a wheelbarrow and get a shovel. Push the shovel in the ground about 6 inches. Break off about 1 foot of sprig or several of them and push them in the hole behind the shovel, put your foot on the sprigs and pull the shovel out. Stomp the dirt down, move about 10 feet and repeat. The long runners are pretty tough with stored energy and moisture in the mature runners, will establish several roots in that 6 inches and will spread quickly. Next rain, repeat in another area of the 4 acres. In a few weeks, you will have a hybrid bermuda nursery to get more runners from. Russell is a variety here that yields well and spreads very quickly.
Yeah, dirty hard work. Sometimes that is required for best results. Too much work to place them every 10 feet? Then go 20 feet or 25. Will just take more time to establish. The hybrid bermuda's like Russell grow almost as fast as kudzu, putting down roots and branching every few inches. Best thing is no tillage required and no erosion. Even if the land is rough, you can wait until the bermuda has spread, then disk it up in the spring to smooth it out and distribute the bermuda. It won't die with one disking and will come back quickly.
The hard dirty work will soon be forgotten when you have a field of productive hybrid bermuda. Its only four acres.
Forget that subsoiler. Subsoiler, bermuda seed and erosion are not compatible. Those little seed require a firm seedbed. Firm seedbed will be susceptible to erosion. Subsoiling across the erosion won't give a firm seedbed needed for germination.