He is just going to get more expensive next year. Either the more expensive tag or the market getting more expensive. It makes zero sense to not go ahead and do the 4 years and keep his yearly average as low as you can.
Yea, the only way it makes some semblance of sense is that they (McCarthy specifically) would rather see him prove it on the field for another 16 games and think paying him more a year from now is worth it.
If they were to give him a 4 year deal averaging 38.5 next year the AAV, if you include the 31 on the tag this year, would be 37 over 5 years.
That's not that big a difference from a 4 year deal averaging 35 this year, but that's only if McCarthy is hell bent on seeing him prove it in his system.
That's quite literally the only scenario that makes sense unless the organization truly doubts that he's the long-term solution.