Improve the hierarchy of H3 and H4 heading styles

[BrieWhyatt]

What is the problem?

Content designers and interaction designers have noticed that visually, there is little visual difference between H3 and H4 styles.
This means that although there won't be issues for those who navigate the site using a screen reader, those who do rely on sight may find it hard to distinguish between the hierarchy of the content

Examples

.

We believe it would be useful to explore making the difference between these two heading styles more prominent, so that sighted users can successfully read the content.

[sarawilcox]

Recommendation from Brie.

See Mural CMS/Nav Team: H4 experimentation

[sarawilcox]

Figma file exploring the issue: https://www.figma.com/file/AIFpsdNn1FmtVSNhbdG2vQ/CMS%2FNav-Team%3A-H4-experimentation-(Copy)?type=design&node-id=0%3A1&mode=design&t=m7WfsHgSwwo7OMjM-1

See Slack for some more comments on this issue: https://nhsuk.slack.com/archives/C05G1JY3WQ6/p1699443589717459?thread_ts=1697627595.270859&cid=C05G1JY3WQ6

[anandamaryon1]

Resolved with the new type scale and merged: nhsuk/nhsuk-frontend#1019