This topic is pure speculation.  If you have not read book 9 yet, I suggest you skip this until you have.  Assuming people can still use the power in the 4th age, and assuming Saidin and Saidar are still around in their third age form, what happens to the things which were done with saidin prior to the cleansing of the taint?  Rand, and a few Aes Sedai, for example, were healed with the power prior to the taint being cleansed.  Is there a chance, then, that the taint will still be around for them?  Also, many men used the power prior to its cleansing so the taint would still be in them.  Might they not still go mad?  The bowl of the winds drew on saidin to fix the weather, but it drew on the tainted saidin.  Just as the ways became corrupted because of the taint, what might happen during the 4th Age?

 

Of course, we will probably never know but this is just something I thought I would throw out there.

I am 99% sure RJ stated that they would be stuck with whatever damage has been done, but they wont get any worse.

Indeed--though remember that that is without intervention. Ishamael implies that the power could have been used to help with the effects of the Taint--some answer may be found.

Ishamael implies that the power could have been used to help with the effects of the Taint--some answer may be found.

 

As a result, it's not beyond the realms of possibility that someone (Nyn?) will figure something out.