I am in the process of finishing my basement and decided to use a Saniflo Saniplus macerating pump unit for lifting wastewater from a sink, toilet, and shower into the existing 4 " drain line (the 4" drain exits the basement approximately 4' from the basement floor). The force main is the easy part, since the drain line runs directly above the new pump unit, but there is no convenient vent stack available to tie a vent line into. Are there venting alternatives (Saniflo says that cheater vents won't work since the unit needs two-way air flow)?

Sanaflo's correct. Unlike your house vent system a holding tank vents both ways. Out when a fixture discharges into it and in when the pump lifts it up to the main. You must vent the tank but you do have options. You can run a vent out the roof or up and revent into a existing dry vent at least 6" over the flood rim of the highest fixture. Another option would be to exit out the basement and run up the side of the building a ways. However there are limitations.
You must not terminate them under a window, door or any ventilation system, or within 10 feet of such openings unless it's at least 2' above such opening. The vent terminal shall be screened and slope back to the tank at 1/8th to 1/4" to the foot. (Chapter 14, Section 1404.4 of the Standard Plumbing Code) good luck, Tom

Thanks for the very fast reply.