internally it is using Toshiba 5400rpm hard disk. performance is same as any other USB3.0 external drives in the market.

2.5" external hdd usually uses 5400rpm hdd due to less power consumption that suits USB 500mA specs, 7200rpm drives usually consumes around 700mA. even if it spins at 7,200rpm, it will still be bottlenecked by USB interface, therefore makes no difference in this aspect.

well if it's a single 4tb (i'm guessing internal drive), then just install into your pc. for things like NAS, i'd only ever recommend 4bay-6 bay at most, and even then using proper NAS hdds, which i doubt yours is. not to mention i doubt you'd want to spend rm2xxx-4xxx just to buy a NAS).

If you want portable storage, you could get an single drive external hdd casing to bring it around with you. But seeing as you only have 500gb in your desktop, i think it's still better use as an internal hdd.

That said, your system looks very ancient. I understand your a basic user.... but....

any chance you'd be willing to upgrade to skylake ? (this is the current pc architecture just released this year)

get the cheapest i7 cpu (if you really want to go cheaper theres i5, but i don't recommend that), motherboard, compatible ddr3 for the motherboard. Budget around 2-3k myr 

PS: your english is fine, i understand you perfectly :} Is okay if you want to speak in bahasa, cause people for sure will know how to speak in our nation tongue