I asked Synology support for HD204UI a few days ago. I also asked about mixing HD203WI and HD204UI in one DS.
I get the answer this morning as following:
Currently this disk is under our testing list and we will purchase it to verify it. Once it's tested we will announce its compatibility. You could subscribe Synology eNews to get the latest update to this kind of news.
Suppose it's supported, you could use this kind of disks working with 203WI disks.
Hope this helps.
--
Sincerely,
Adair Hsu
I hope... they will buy it soon

BTW: As you probably know, F4EG have 667GB platters instead 512GB in F3EG. It means HD204UI will have 3 platters only instead 4 platters in HD203WI. It looks it can be more silent and more cool then not so silent previous F3 model - HD203WI.
