Deep Plane Facelift - is IV sedation for 4 - 5 hours safe?

If you are in good health with no issues, it should be no problem undergoing a procedure that long under sedation or general anesthesia. The more important question would be how safe is the facility? Is it state licensed, Medicare certified, uses board certified anesthesiologists, etc. If all of these answers are yes, then you should be in good hands.

I think 4-5 hours is probably fairly typical/average for that type combo of procedures. It is safe but it would of course be very specific to you with your medical comorbidities and what they plan to give you for your anesthesia. Local is possible but may be challenging for the combination of procedures you are getting.

It is close to unfeasible to perform a deep plane faclelift with Connell's flaps on the SMAS, Guerrerosanto's platysmaplasty, defatting, skin adjustment, etc under local and sedation in 4-5 hours.

A skilled and quickhanded surgeon needs 5-6 hours, no less, and general anesthesia is a must.

I fear in the end you won't get a real deep plane facelift.

We do those procedures together often. It takes about 3 hours or 3 and a half. We use IV propofol and it is a type of general anesthesia that is very safe. In fact, that's what most surgeons would want for themselves.
Having done literally thousands of cases this way in our surgery center, I can tell you it is very safe and comfortable.