Cheapest Western Digital 4TB GREEN internal / external HDDs I can find (delivered), from Futu_Online on eBay with the 20% off eBay coupon.
Use the eBay coupon CTREAT20 at checkout for 20% off: https://www.ozbargain.com.au/node/208273
External WD 4TB $176 delivered.
Internal WD 4TB $184 delivered.
Don't forget CashRewards for 2.5% cashback!
You can certainly find cheaper Seagate same-size drives out there however they will be using SMR technology resulting in lower overall performance. This deal is for anyone specifically after WD drives.
edit: It has been brought to my attention that the 4tb Seagate comparative mentioned is in fact not SMR. Still a great deal for the WD-inclined! (know your stuff)
Wtf. Why wouldn't you just but the external and rip the HDD out for an internal?