Ok! So having looked into this to answer my own question, it would seem Stretch users installing the Mosquitto extension for PHP since March 2017 using the command pecl install mosquitto-alpha will actually have version 0.4.0. You guys should be able to confirm that is the case as it should be listed in the last section on the admin page of emoncms or by using pecl list-upgrades.
By specifying -alpha it defines the acceptable release level deemed acceptable as “alpha”, ie any stable, beta or alpha releases are allowed, the default unless overridden in this way is “stable”. Since there has not yet been a release tagged as “stable” we have to use a -alpha or -beta suffix otherwise the install will fail, stating there is no stable release available.
So whilst there may arguably be some benefit in using mosquitto-beta to avoid potentially buggy “alpha” releases, I don’t think any change is needed except to be mindful that we may not all be running the same version when debugging and to possibly include checking for upgrades once in a while rather than just blindly sticking with whatever was the release at the time of initial install, just in case it goes to a “stable” release.