Can I re-enter Thailand in December after two previous entries this year?

No one can really tell you. It works, until it suddenly doesn't work. Just make sure you have a backup plan.

Nobody can answer your question other than the immigration officer you will stand in front of. A new year makes no difference

Just get the right visa

I have entered 4 times in a year with at least several weeks between visits and flown long international distance with a return ticket. Was never questioned

Keep us informed as to what happened

It would matter whether you were there in the beginning of the year, i.e. have by now been away for a decent amount of time, or you've just recently completed those four months and haven't really been away just yet. In the former scenario, it would be fairly safe to assume that you will get in. In the latter, the odds may still be in your favor, but it might be smart not making yourself too dependent on a positive outcome.

Chances are, you’ll be questioned and most probably strongly recommended to apply for a proper visa next time you’re entering the country.

There's NO longer a 2 times per calendar year entry into Thailand using the visa exempt entry.

You already have been in Thailand this year for 4 MONTHS. Sounds like you entered twice and stayed each time for 60 days without doing a 30 days extension of stay.

When were the 4 MONTHS? In early 2025... or recently?

Are you Married to a Thai or Father of Thai child?

What is your passport nation?

Be prepared for questions from Thai Immigration Officers upon arrival into Thailand...

...Have proof of your flight ✈️ departing Thailand within the 60 day STAMP you will receive...

...Booking address of your initial week of stay...

...At least 20,000 Thai baht in CASH or equivalent currency to show if ASKED.