P4s aren't supposed to be counter to Jackson (like atgun are supposed to counter P4j), and again it's long range penetration, P4 like panther have already 100% to pen at low range in case of dive.
And its not counter to jackson, the same way T34\76 arent supposed to counter Elephant. But if you flank Elephant or if you managed to get close to the Jackson then you can counter them with mentioned units.
Long range with P4 is 40, Jange of Jackson is 55, long range for P4 is close to a medium range of Jackson, if you let P4 get so close, its really your fault because you have huge range window to kite with it.
Calliope don't have panther or damage engine mines to protect them. If a panther dive, then you have to keep your jackson at distance and the calliope because there nothing that can temporize your escape.
For mines its true, but you still have snares on your riflemen, you still can lay mines with RE to slow dives, one of the cali comms have ranges with elite zooks.
Its not like you dont have anything but jackson to protect your calis (while jacksons are still the most effective) and cali survivability was nerfed to begin with because it requred less protection and was more forgiving on the unit which is supposed to be protected and used on range.
And unlike any rocket arty in the game, it cant be killed with counter-arty and it cant be sniped if its too close, its doc unit sure, its more expensive sure but its still by far the best rocket arty unit in the game survivability wise, and when it was able to bounce and had 3 shot-hp its survivability was just over-tuned.