Single-flow speed of nBase-xx4 links (was: SFPs + Fiber = cost more than switch?)
"So, that means that the highest possible speed for a single connection is 100Gb?"
No, you get 400Gbps. The nBase-xx4 interfaces run four "lanes" of ethernet symbols. The symbols are round-robined between the four lanes. An ethernet symbol is 64 bits logical, 66 bits on the wire (to allow for clock recovery).
If you are thinking that this means the media carrying the four lanes needs to have exactly the same latency then you would be correct. This is conveniently enforced using fibre assemblies and connectors with multiple fibres.