A quick, maybe imperfect solution – use an expression in the `volume` filter, e.g.

    volume=1-0.85/(10-4)*(t-4):eval=frame:enable=...

For t=4, the volume will evaluate to 1, for t=10 it will be 0.15:

    for t=4:    1 - 0.85 / (10-4) *  (4-4) = 1 - 0.85 / (10-4) * 0 = 1 - 0 = 1
    for t=10:   1 - 0.85 / (10-4) * (10-4) = 1 - 0.85 = 0.15