I came here looking for a noise comparison between ST4000DM000 and WD40EZRX, WDs current green 4tb.
I own 8x Seagate 4tb and to me they are pretty good on my server, but i also dont have it near me nor i run a raid setup, the starting gets loud though, but after it loads up and only 1hdd is access to me its acceptable for none suspended hdds. There are other users on the forums that also had good experiences with them, weather or not they are better or worst than WD reds 4tb idk, usually WD greens and now reds are the standard for SPCR users, and even the recent reviews backs it up being a very quiet drive.
SPCR did review the Seagate 4tb but the NAS version, Seagate NAS HDD 4TB ST4000VN000-1H4168
, in there you can see a comparation to the WD reds 4tb, given its not the same drive that you are looking, thats the closest you might get.
I recently bought a 3tb WD Red, and also pretty good, i cant hear it inside my synolongy diskstation, pretty good drive imo. I do have my doubts on the 4tb, back when i bought my seagates there were no 4tb reds, so i didnt have the trouble of chosing in between, now im wating for 5tb reds (that should been released on dic 2013) and seagates (if they releases them on 5900rpm), to upgrade my 2tbs, but for now im happy with both companies.