Frankfurt Germany 4-5 star Hotel early December.

Depending on which train station you are referring to, I can point you to a couple of good options at the Airport train station. There is a Marriott and a Sheraton hooked onto the airport and the walkway to that train station. This is not the "main" station but is very well connected and is only a stop or two away from the main station. If you want to be in town and/or by the main station, they you will need to look in the RS book or perhaps others will reply with recommendations.

Try the Flemings, Hotel Nizza, or Hamburger Hof by the north exit if you want close to the train station, though honestly, you can take the S-8 or S-9 from the Hauptwache or Konstablerwache to the airport if that is what concerns you. If you want to go to the Christmas markets, the central locations are better than near the train station.
Try the Hilton, the Westin, Steigenberger Frankfurter Hof, Flemings City Center, or Ruby.

Being a person who never pays attention to hotel stars and having heard (perhaps falsely) that they had to do with the types of amenities rather than the quality of them, I did a little digging and found this website for the rating system in some European countries, the European Hotelstars Union: https://www.hotelstars.eu/

You can find a basic list of the criteria for the number of stars as well as a link to the complete descriptions here: https://www.hotelstars.eu/criteria/

Scroll down this link for Germany to see a "Quick Search" feature that is actually a linked list of cities and regions: https://www.hotelstars.eu/de/deutschland/service/hotelsuche/

Note that although listed, not all cities and regions may have hotels listed. I'm not sure if that’s because the ones I tested were being updated or if there's a glitch in the system. If it happens when you click on the link, use the search boxes above. I tried that and got results that didn’t show up using a link.

I definitely learned a lot from that little bit of research. I hope you find a good place for you to stay.

I would trust Ms Jo on anything Frankfurt related.

Ms. Jo is hinting at locations in the center rather than at the station, and I do think that they would be a better choice given how easy it is to get to Frankfurt Hbf. from these locations. The station neighborhood holds little of interest, and while its reputation is perhaps a bit overblown, it can indeed feel shady at times.
If you look at a map, you can see a ring of boulevards and parks corresponding to the former city walls: I would stay inside this ring. This is where most of the fancier hotels are located, anyway.

slightly further out, a very easy S-Bahn, U-Bahn or tram ride to the Hauptbahnhof is the Maritim Frankfurt. I stayed there very happily a few years ago and the breakfast was fantastic. As are the views from the nearly rooftop swimming pool.. I found nothing negative about it.

It is 4 Star

https://www.maritim.de/de/hotels/deutschland/hotel-frankfurt/unser-hotel