I didn't catch that you had a 4" vent pipe. A 2" vent/siphon break will be plenty for for a 6" siphon and will make it easier to begin siphoning. 2" check valves are about $10. Check valves aren't 100% reliable so you'll want the check valve capped off and only remove the cap to attach a vac to manually start.



"The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge." Stephen W. Hawking