doc,n-api: update matrix for N-API version 4

The N-API version was bumped to 4 for Node.js 10 in
https://github.com/nodejs/node/pull/25633

Refs: https://github.com/nodejs/node/pull/25633

PR-URL: https://github.com/nodejs/node/pull/27832


Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>