How much screws for 3.5" HDD? 4 or 6?
The 3.5" HDD default design for 4 screws, due certain HDD need extra inner space for more disk platter and move 2 tradition screw holes location to the edge, thus some new HDD tray have this new screw holes only, for compatibility some new HDD & tray place tradition & new screw holes both.
And following WD & HGST specification about screw numbers for your reference:
WD EX1200M-2 6TB TRM 2679-800080-A00.pdf (enclosed)
page 36, section 6.5.3 Drive mounting
https://www.hgst.com/sites/default/files/resources/Ultrastar_7K6000_SATA_512n_OEM_Specification_Rev1.0.pdf
page 39, section 6.5.3 Drive mounting
http://support.wdc.com/knowledgebase/answer.aspx?ID=9915