At the moment H3 (1.375rem) is smaller than H4 (1.5rem) on mobile. Ideally we should use values from the list of fonts that we have defined here: https://github.com/greenpeace/planet4-styleguide/blob/master/src/base/_fonts.scss

Font sizes (from Will's comment): 
H1 = XXXXL : 4rem. (mobile) XXL : 2.5rem
H2 = XXL : 2.5rem (mobile) XL : 1.75rem
H3 = XL : 1.75rem (mobile) lg : 1.375rem
H4 = lg : 1.375rem (mobile) md : 1.25rem
H5 = md : 1.25rem (mobile) s : 1rem
H6 = s : 1rem (mobile) s : 1rem

tasks

• Apply the new values and check if this works with other css.
• If  this affects other css but result is still acceptible, open a follow up ticket.
• Fix an other css that would be really broken by this change.