100G-LR4 is a technology that's on its way out, and this illustrates why. Muxing four 25G lanes is complex and costly and these days also unnecessary at 100G.
100G is going single lambda.
This is still the way to build 200G/400G/800G optics, though.
This is still the way to build 200G/400G/800G optics, though.
The QSFP-DD modules are also bigger (they stick out further from the slot) and have more power available, so that probably helps somewhat with engineering.