The majority of modifier are Multiplicative, I suspect those are also although I did not check.
I did a few more quick tests of different abilities stacking: T4 bonus, 'Counterattack Tactics', and 'Assault and Hold'. It seems that the bonuses are applied multiplicatively. So, for example, with T4 and 'Assault and Hold', the math is:
100% (default capture rate) x 125% (T4) x 200% (AH) = 250% de/capture rate
Full testing:
T4: Increases the capture and decapture rate of all Ostheer infantry units by 25%
CA: +200% capture rate
AH: +100% (de)/Capture rate
default
decap - 10s
cap - 15s
counter-attack
decap - 10s
cap - 5s
assault and hold
decap - 5s
cap - 7.5s
t4 default
decap - 8s
cap - 12s
t4 + counter-attack
decap - 8s
cap - 4s
t4 + assault and hold
decap - 4s
cap - 6s
By far, 'Assault and Hold' has the best synergy with the new bonus.