is it ok to use my ipad charger for my iphone4?

Yes, but not the other way around. The iPad charger presents more current, which the iPad needs. It is plenty of current for the iPhone or iPod. (Don't worry, the device will only take the current needed, you won't burn it up or anything!)

The smaller chargers provided with the iPhone and some iPods do not have enough current for the iPad.

If you have both devices then it is best to get an extra iPad charger or 2 and then you can use it on any of your devices.

Actually, you can safely use an iPhone/iPod Touch AC adaptor to charge an iPad, but the changing will be much slower than with the iPad charger. The effect will be the same as using low current USB ports found on many older computers.

Agreed. I tried it, and the little iPhone charger could not supply enough current to charge the iPad. It might even damage the iPhone charger.