Reallocated Sector Count Increasing Daily

Hey All

I've got an NV+ with 4 x 2TB drives in it.   I'm getting daily Logs showing the reallocated sector count increasing on DIsk 4 which would seem to indicate it may be close to failing.   My question is if I replaced it with a HDD of higher capcity, would it increase the overall storage capacity of the NAS.   I know that with the raid configuration a higher capacity Drive will be restricted to the sized of the smaller drives but the underlying question is this:   If I replace 1 HDD with higher capacity today, then replace the other other 3 at a later time, will they all expand to their full potential or will they remain restricted to the original capacity of the existing drives?

Thanks!