Never trust what a seller says. I know it sounds harsh..but you really can't. I went to see this one Arab x Dutch Warmblood gelding who was supposedly "beginner safe" and "sweet". He was a monster! Pushy on the ground, tested you, greener than grass, wouldn't pick up a trot to save his life, etc.. Basically, the exact OPPOSITE of what I wanted. Sure I handled him fine, but he was just a pain! Anyways, the owners seemed very kind yet lied to my face. He even reared up when he saw a dressage whip..
You CAN get a 4 y/o...do I advise it? Not unless your an intermediate rider with a trainer or an advanced rider. And be honest..if you're a beginner..absolutely not. Anything younger than 8 generally acts like a big baby (there are odd balls though)...I have had quite a bit of experience with young horses.. And being 13, you can't have been riding long..so I just don't advise it.
Although, if I were you, I'd go out and try out this 4 y/o anyway lol
It's ultimately up to you!