It looks like Mikrotik never implemented an oid for fan3 and fan4, at least not directly. There are pretty old discussions on that in their forum.
Instead of seems they provided a “gauge table” oid (.1.3.6.1.4.1.14988.1.1.3.100) to address a number of limitations in SNMP monitoring. It was added in this release: v6.47beta [testing] is released! - MikroTik
So they put a bunch of device health values here together rather than in their branches of the oid tree. And some of the values duplicate what is available elsewhere (fan 1&2 speed, voltage, and a temperature value or two). I don’t know why they would do it this way, but the mib from Mikrotik includes these oids already (as “mtxrGaugeTable”).
It seems like the most straightforward way to achieve your goal of monitoring the fan speeds might be to add custom oids in the device settings and graph/monitor/alert on that. It won’t be as nice as the result you are chasing in your approach, but it should work pretty easily without fighting the limitations of their implementation.
Sure. Will deploy this at my own instance during this week. And I have now deployed this at my own instance. Instead of my own ugly hack (PR15281). Will provide feedback after running with the patch for a few days.