I definately wouldn't leave a 14year old overnight with a four year old, but assuming they are mature, capable of coping in an emergency, doesn't normally fight with sibling, can get hold of you or a trusted neighbour very quickly - I'd certainly think it would be OK for a few hours.
I babysat from when I was 13. Stuck signs in local shops. Most parents would get me to come around during the day to meet and play with the kids.
But I remember one family asked to to come around at 6.00 (1st time ever there-I assumed it was to meet the kids). They left. No phone numbers, kids had never meet me before, no instructions on bed routing etc and they didn;t come back until 2.30 in the morning & let me walk home alone (about a mile away). Never babysat for them again & I got very clear about what I expect from the parents in future!