Update the Prototype Kit to use GOV.UK Frontend v4.0.0

[vanitabarrett]

What

Once GOV.UK Frontend 4.0.0 has been released to npm, update the Prototype Kit to use it. As this is a breaking release, we might want to generate a pre-release ahead of the actual release, so we can test updating to v4.0.0 in the Prototype Kit and prepare any changes we need to make in advance.

Why

So that the Prototype Kit reflects the latest release of GOV.UK Frontend

Done When

• Create a draft PR in the Prototype Kit using a pre-release of v4.0.0
• Make any changes required in the Prototype Kit in order to update and raise a draft PR
• Draft PR updated once v4.0.0 published to npm and merged