Is 3G network always available when 4G network is?

I'm going to buy an Android phone which has unfortunately blocked 4G calls by the operator. Only 3G calling will be possible. I think it's enough for voice, but: Since I travel a lot, is the 3G network available everywhere when 4G is? Or is it completely different technology so I might end up in the places where only 4G signals are available, but no 3G?