I personally think the Jagdpanzer IV is fine, it's a damn good unit with great veterancy. ...
vet bonus: (if I have not missed anything)
Jagdpanzer IV/70 (V)
1) Unlocks the 'Cautious Movement' ability
2) +25% HP +20% accuracy
3) +20% rotation speed, +20% speed, -10% reload
4) -15% reload, +15% accuracy
5) For first shot fired out of camouflage x125% damage & x125% accuracy & x125% penetration
for comparison reason:
SU-85 Medium Tank Destroyer
Unlocks the 'Tracking' ability
+30% penetration, +30% accuracy
-20% reload, +20% rotation speed, +20% speed, +20% ac/de-celeration
M36 'Jackson' Tank Destroyer
Unlocks the 'HVAP' ability
+30% accuracy, -20% reload
-15% reload, +30% penetration
Sherman Vc 'Firefly'
acceleration + 15%
-25% reload, Weapon rotation+ 35%
Accuracy +30%, +40 Damage
So imo a vet 3 JP 4 has inferior bonus to a vet 3 SU-85 to M36 I would even go so far to say even to FF.
(edited)