If you took up this offer it should start straight after the 12th of January.
You could also try cancelling the Sports Membership to see if you get a better deal (though i would wait closer to the 12th of January, maybe two or three days before).
Even if you cancel you can still watch up until the 12th of January.
My Sports Membership is in it's last 30 days and at the moment i am not sure if i am going to extend it yet because of the Omicron Covid variant is spreading rapidly and may have a major effect on the sports schedule.
if you did decide to take up this offer which would automatically kick in on the 12th of January and say the Omicron variant or another Covid mutation had a major effect of cancelled sports events during your 4 month offer period and you decide to cancel in month 1, 2, or 3 of the offer because it isn't worth it.
Then you won't have to pay the months left on the 4 months offer that you don't use.
I think i am going to give it another week or two before i decide myself (my Sports Membership is due on the 13th of January) and see how this new Covid variant is having effect on the sports schedule.
I am sure there will be some new Sports deals in the offering in January.