If you have that much runway, you should apply to Recurse Center https://www.recurse.com/ which will give you a chance to build up your skills, your confidence, and your network.

> Will the 4-year gap and my choppy CV be a major hindrance to my hirability?

Yes, so spend some time thinking about how to get a hiring managers attention. If there's nothing separating you from someone with more recent experience you're going to struggle and get filtered out before anyone talks to you.

Ideally you can find a way to have conversations with a hiring manager or someone on the team you'd like to join vs emailing in a resume that a recruiter is going to toss. Find or create a reason for them to want to talk to you.