It's not. It is to my belief that the enthusiasm as it gets closer to 5 it's momentum decreases. So at 13 it drops faster per day than at 9. Thus I think the same thing happens when your below 5. I was in your exact same situation this season and had to use a TIE to finally get above 4.
In addition, your team's enthusiasm tends back towards the default value of 5 over time. The further away enthusiasm is from 5, the more quickly it will tend back towards 5. Enthusiasm will always reach 5 during the offseason.
It's actually in the Game Manual:In addition, your team's enthusiasm tends back towards the default value of 5 over time. The further away enthusiasm is from 5, the more quickly it will tend back towards 5. Enthusiasm will always reach 5 during the offseason.