You have discovered the difference between static and dynamic thrust. The higher pitch but lower diameter props give lower static thrust, but higher dynamic thrust.

Dynamic thrust is the thrust generated when the prop is already moving though the air. As the plane accelerates, the dynamic thrust decreases, but decreases less on higher pitch props. Think about it - a prop with a pitch of 8 will try to fly the plane twice as fast as a prop with a pitch of 4 for a given rpm. Therefore at the max speed of the 4 inch pitch prop, the 8 inch pitch prop is still trying to accelerate, so is generating thrust.

So in the hover, as your plane accelerates upwards, the dynamic thrust drops very quickly with a 4 pitch prop, so quickly stops accelerating.

Try different props, I would go with a 14x6.