IPhone 4 sim locked, same carrier?

I have two iPhones here, an iPhone 4 7.0.2 and an iPhone 4S 6.1.3
both are not jailbroken and recently i switched the contents of the phone and switched sim cards as well. Both sim cards are from the same carrier and both are being payed monthly (post-paid)

when I switched sims, the sim i used on my 4s is locked on my iphone 4, but the sim on my iphone 4 is okay with the 4S. Again, they are of the same carrier.

How do I fix this without jailbreaking?

It sounds like your iPhone 4 alone is locked to carrier. You can resolve this using remote unlocking method that will be available at any vendors online like Mobile-unlocker.com.Once unlocked you can use any carrier to it.