Iím no expert in Thiele & Small Parameters so donít hold me to anything below. I just cut and paste the AudioFrog GBD2 vs D4 perimeters below for comparison sake. Iím guessing the major differences are in the voice coils themselves where the D2 has a shorter and lighter voice coil and the D4 has a longer and heavier voice coil although the Mmd says otherwise. This also explains why the D2 has slightly less XMAX with the shorter voice coil. If the voice coil and weight are less on a speaker than the sensitivity should go up and you sacrifice some bandwidth as a trade off. The D2 wired at 1 ohm has a sensitivity of 95 db at 2.83 volts/1meter and the D4 is 92 db wired at 2 ohms. The inductance is also greater on the D4 and again Iím guessing due to having a longer voice coil. Also attached is a picture of the model in BB showing the predicted response based on these ts parameters. Some things to note are that not all companies share these differences between D2 and D4 drivers of the same models and a lot of them only post the series parameters or the ones that look best to the misinformed so it can be difficult to get good information.

Thiele & Small Parameters:

Revc: 1 Ω or 4Ω
[email protected]: 0.33 mH (2 Ω configuration)
Mmd: 183 g
Mms: 189 g
Cms: 153 ĶM/N
Vas: 57 liters
Fs: 29.5 Hz
BL: 8.37 TM (1Ω configuration)
Qms: 8.44
Qes: 0.52
Qts: 0.49
Xmax (1-way linear): 17.6 mm (11/16Ē)
Basket Displacement: 4.17 liters (0.147 ft3)

Thiele & Small Parameters:

Revc: 2 Ω or 8Ω
[email protected]: 0.58 mH (1 Ω configuration)
Sd: 510.7 cm2
Mmd: 177 g
Mms: 184 g
Cms: 171 ĶM/N
Vas: 63.7 liters
Fs: 28.4 Hz
BL: 11.47 TM (1Ω configuration)
Qms: 8.82
Qes: 0.52
Qts: 0.49
Xmax (1-way linear): 19 mm (11/16Ē)
Basket Displacement: 4.17 liters (0.147 ft3)

In the picture of the graph the D4 subwoofer has a slight advantage below 48 Hz but above that the D2 is louder by a significant amount with less power.