4g won't technically affect 3g at all. Different type of network.
But, of you're in a 4g area, and a significant amount of people get 4g phones and use only 4g, then the 3g network won't be as crowded.
i think it will provide a benefit for the OV, the OV isn't a 4G devices, it can never get anymore then medicore 3G but Virgin's new 4G network it will gets users off of the already bogged down 3G network.