Yes it can. On the App Store in iTunes, Voice4u will be tied to the iTunes account you make the purchase from. If you own multiple devices, for example, an iPhone and an iPod touch, you can sync both devices to this same account and use Voice4u on both the iPhone and iPod Touch.
Note that since you can only sync a device to one account at a time, Voice4u will be removed from the device if you subsequently sync the device to another iTunes account on another computer. To put it back, you will need to sync again to the original iTunes account you made the purchase from.
Also read a tip about copying Voice4u data on more than one iDevices.