G38.2 (what gSender uses for probing) means that the GRBL firmware expects that the pin isn’t triggered at the start of a probe cycle. If you are getting an alarm 4 right as you initialize a probe cycle before any movement, that means GRBL thinks the pin IS triggered, and is likely some variation of the following:
Check your EEPROM $6 setting (Invert Probe Pin). By default, it should be $6=0. If it’s $6=1, change it to 0, the toggled off state. You can either use the console for this ($$ to see EEPROM settings) or the firmware tool.
Check the status report in the gSender console by sending a status report by typing “?” with no probe continuity (so the probe block is not in contact with the router bit). You should get a report back similar to “<Idle|MPos:0.000,0.000,0.000|FS:0,0>”. If the report back also includes a “P:NP”, that means that GRBL is reporting the pin is triggered. It might be your ground and probe wires are swapped, dirty, or shorted, and you can try to fix that.
Noise in electrical line. Make sure your probe wires are away from all other power/stepper wires and try again.
Try these steps in order and let me know what the result is. Probe cycles not running for this particular error is almost always an issue with the physical installation if it’s not the firmware option.
You should be able to probe 25mm without issue. If you’re getting an error 4, it probably throws the error as soon as you start the probe sequence, correct?
Sounds like you might have some sort of electrical issue… Hard to say exactly what the issue is.