DR4 = 4 times the fiber requirement of FR4 (shorter distance). This is why POET is building both LR4 and FR4 which effectively covers all data center reach requirements  in the most economical way possible. And as I recall QSFP-DD is a modification of the 200G QSFP standard (which was stop gap) with added components (gearbox) to double the speed.

Because POET's FR4 which has the usual benefits of the OI platform also utilizes waveguide filtering it will create a new economic model that will basically undercut DR4 and justify implementation in distances now utilized by that standard.

And finally it is important to remember what you have learned about the benefits of heterogeneous integration and why there is now a roadmap for all aspects. It the way of the future because you need to combine the best material sets to achieve the best products. POET's/Suresh's design uses all the benefits and optimises them in a very clever way that will be the basis for many new generations. 

https://eps.ieee.org/technology/heterogeneous-integration-roadmap/2019-edition.html