I have not had any issues using all four slots on the Z400s with GPUs.

A large portion of my systems are 4x Geforce 1060 -- with one card in the bottom x16 slot and risers on the other three. As pictured:

https://thumb.ibb.co/bSmqxx/0316180942_resized.jpg

https://thumb.ibb.co/cvKojc/0316180942a_resized.jpg

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The splitters also do seem to work fine as I have a few systems with 6x 1060 or 1x 1060 / 5x 1050Ti using splitters.