vidhubhushan
Alakh Niranjan
"External connector" like those usb enclosure cases are a big no(no ventilation so case running internal hdd gets hot quickly) but open type hdd docks are acceptable provided room temps are decent & your power supply via ups is good(forget about directly connecting external hdd power connector to wall socket unless you don't care much for your data). Go with backup plus 4TB this time & sell the refurbished replacement WD drive you will get on olx/erodov/techenclave forum for ~4.5-5k.
P.S. I will also suggest not to order internal hdd online as they are very delicate & no matter how good the packaging is,there is still chance of damage(not visible immediately but rather long term which I suspect is the case with your current 4TB hdd) given how courier companies handle the products usually.
i have an old tech-com usb power connector with power adapter but don't know if it is going to work with 4tb or not. i will get external one and later try to get a 4tb internal along with or after new computer. thanks dear.