Would I be able to have Renuvion for additional tightening 4-years after my facelift?

Yes after a facelift, Renuvion is great way to to help with additional tightening, without having to undergo a repeat facelift. Would have to see you in person to make the final determination, but it sounds like you could be a great candidate!

you will need to post photos, but yes Renuvion and facetite can be used post face lift along with threads, lasers and microneedling RF to give improvement or prevent more sagging.
With significant neck sagging you would need a neck/face lift with muscle tightening (platysmaplasty). at the same time lasers (fraxel,  co2 or erbium( or microneedling RF (fractora, intensif) can be . performed to improve the crepe skin or wrinkles.  You need a formal consultation, as the neck can be treated with a combination approach:
1. micro needling and lasers for texture
2. botox, disport or xeomin to help platysmal bands
3. intensif or fractora RF microneedling for crepe skin and lifting
4. fillers like belotero, restylane silk, juvederm volbella to improve lines and folds
5. neck/face lifting if sagging is prominent or early starting +/- platysmaplasty (muscle tightening)
6. J plasma/Renuvion, thermiRF, inmode facetite for internal tightening
7. ultherapy and thermage for external tightening
8. regular Venus legacy, exilis ultra, or inmode forma treatments with facials to maintain tightness
9. PDO novathreads or silhouette instalift cone threads for lifting and long-term collagen production
see an expert for a full evaluation and treatment plan based on your individual situation and goals.
Best, Dr. Emer.

It is hard to answer your question without photos or a consultation. That being said, you likely can get this done, but the results may not be as pronounced as your facelift procedure.  A Skype or FaceTime consultation with a board certified plastic surgeon will help you get an accurate recommendation and pricing.
Respectfully, 
Arian Mowlavi MD, FACS
Realself 100 Hall of Fame Inductee