How to stop iPhone 4 contacts being automatically replaced by an iPhone 6 contacts with the same Apple ID logged in?

So my phone got damaged and I sent it for repair. This is not the issue.
Meanwhile, I'm using my Dad's previous iPhone 4. I placed the SIM card in and used a carrier app to transfer all my contacts. After my contacts replaced the one on my Dad's previous phone, he informed me that the contacts on his current phone iPhone 6+ had changed to mine to and he had lost all of his contacts.
Unfortunately he has yet to back up his new iPhone 6+ but I did managed to find a backup of the iPhone 4 stored on my computer.
Here comes the problem: I restored the back up from my computer on the iPhone 4. All the contacts were back. However, whenever the iPhone 4 has wifi on, the contacts will automatically get replaced by my contacts again? I have tried toggling iCloud here and there but nothing seems to be working.
Anyone knows the problem here?

Okay, as I'm not very clear how iCloud works, I'll try my best to explain what I tried to do.
Firstly, what I'm trying to do is to get the contacts restored on the iPhone 4. Once I've got them back, afraid that they might get replaced, I put the phone on airplane mode and turned off iCloud Backup. The iCloud 6 on the iPhone 6 is turned on and is connected to wifi. By turning the iCloud back on with the iPhone 4, I assumed that the replaced contacts on the iPhone 4 would overwrite the ones on the iPhone 6 but it seems to be happening the other way round.
Objective: Trying to get the restored contacts on the iPhone 4 to the iPhone 6.

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