yes its mind boggling indeed.

and poet can. see here:

FR4: Most difficult combination of Cross talk and BandwidthPOET Filters meet the stringent FR4 requirements with wafer scale integration capability

https://poet-technologies.com/docs/agm2019/AGM%20Meeting%20Presentation_FINAL_Web.pdf

slide 23

and in the summary:

Well positioned to participate in large, high-growth datacom and telecom markets

•400G TxRx (DR4 & FR4), 100G TxRx & 5G Wireless

btw it s a whole new thing to review that presentation and that hint for hopefully significant NRE in first half of 2020 catched my eye.

and arista can tell us what those dr4 and fr4 things can be used for:

https://www.arista.com/assets/data/pdf/Datasheets/Arista-400G_Optics_FAQ.pdf