The documentation [0] page for 4.16 shows that Ignition version to be used is 3.2.0. The information was in a different place prior to 4.16, so 4.15 and 4.14 have it at [1] and [2] accordingly. But this is in contrast with what (we?) say at Butane upstream project page [3], where (we?) declare RHOCP 4.14+ to be using 3.4.0 Ignition version. I've opened other different bugs about documentation showing wrong information on Ignition version, see [4], and [5], and there I've assumed the [3] to be correct information. This might or might not be the case though.

4.14+

Check documentation pages for RHOCP 4.14+ and check Butane Project page

    1. Compare the information contained in pages as per description
   

3.2.0 is mentioned in the main MCO page    

3.4.0 should be mentioned in the main MCO page

[0] https://docs.openshift.com/container-platform/4.16/machine_configuration/machine-configs-configure.html
[1] https://docs.openshift.com/container-platform/4.15/post_installation_configuration/machine-configuration-tasks.html#machine-config-overview-post-install-machine-configuration-tasks
[2] https://docs.openshift.com/container-platform/4.14/post_installation_configuration/machine-configuration-tasks.html#machine-config-overview-post-install-machine-configuration-tasks
[3] https://coreos.github.io/butane/config-openshift-v4_14/
[4] https://issues.redhat.com/browse/OCPBUGS-38910
[5] https://issues.redhat.com/browse/OCPBUGS-38909