The h3 is one full step up the scale.
The h4 is bottomed out at body copy size. Only its gap
above separates it from the h5 below.
The h5 is also body copy size.
The h3 is still one step up the scale.
The h4 now sits at a half step, and is visually
balanced between the h3 and h5.
The h5 is body copy size, and now clearly a step below
the h4.