My first time horse?!?!?

Go and see her. Sometimes people lie in their ads and 4 years isnt too young for a first horse. Mine was 2 :) though I had been around horses for 9 years. but when you go see her show up early to make sure they arent doing anything to her before hand. Ask the owner to ride her first because you want to see how the mare reacts to rider and to be able to see her gaits. If the owner objects or seems hesitant, you know you have a problem. And it sounds like you know everything you need to!