This could be a consequence of the games release schedule slipping out of November/December to the first quarter, with the 3rd party yearly offering still releasing in september/october. If the trends continues over Q1, it might mean the PS4 sales are loosing momentum though.
Nope, this is a consequence of not having a lower priced SKU in the market in December. Simple as that. The PS4 sold better all year than any other year in 2017, and went on to have the best BF in PlayStation history and the best November sales outside launch in PS4 history. All because for one week it had stock of a $199 SKU.
This has nothing to do with games or momentum. Its pricing.