What does acidity of para-alkyl substituted phenols depend on? Problem

Arrange the following organic compounds in decreasing order of acidity.


Answer

 S, R, Q, P.

Question
According to my knowledge the answer should be P, Q, R, S. As we proceed from P to S we are increasing the number of methyl groups, and so the number of electron donating groups increases, decreasing the polarity of of the C−O bond, which is the opposite to the answer. This obviously means that whatever I'm thinking is the right reason is the wrong logic, but I'm not sure how.
The reason provided by ML cannot explain the difference in acidities since the +I effect doesn't come into play normally for the para positions.
A more likely reason is hyperconjugation (or no bond resonance) where the number of hydrogens directly attached to the benzyl carbon decreases as it goes from P to S.
This means that the number of destabilizing structures that are produced via this resonance decreases as you go down the list. This leads to greater stability in the deprotonated form compared to each other.
Therefore the correct order becomes S, R, Q, P.