A frame represents a division of time, therefore there is information between frame 4 and 5.
I don't remember in what context I said that before (and I'm to lazy to look it up now) but I will say it again:
The design of a software shouldn't be about the most accurate definition of whatever term, it should be about easy to read information for the user. Take away the struggle of keeping track of all the informations as much as possible and give the user the space to breath and to focus on his work.
Maybe your definition is the most accurate but the visual feedback McNistor suggested is just easier to read. No one can ever misinterpret what he/she sees in the timeline then.
At the end nobody wins a price for term accuracy but for a software that is easy and fun to use.