Why is my fourth LED not lighting up?

The upper ribbon cable is what controls the functionality of those LEDs. While the cable may seem fine, it's quite fragile- especially with how it's folded into the Joy-Con. To confirm general integrity of the cable outside of that singular light, test your rail inputs inclusive of SL, SR, and the SYNC button.

For SL and SR, you can confirm functionality proper either through the Controller Menu or the Test Input Devices feature. For the first option, from the Home Menu, go to the Controllers Menu and select Change Grip/Order. By pressing both SL and SR simultaneously, you should be able to pair the controller. If you can't, one or both of these buttons are likely not functioning. You can get more particular confirmation with the alternative method. From the Home Menu, select System Settings and scroll down to the Controllers and Sensors section. Here, you'll find the Test Input Devices feature. Confirm that both buttons are working through this feature. If one or both buttons do not work, this indicates the possibility of damage to the ribbon cable.

For confirmation of the SYNC button's functionality, simply hold the button down for a second while the controller is off, and you should see the LED lights cycling. If the SYNC button is unresponsive, then it indicates damage to the relevant ribbon cable.

If all of these inputs are working properly and your Joy-Con is otherwise functioning normally, then you could have very slight damage to either the cable or the specific LED assuming that the ribbon cable's connector is properly seated. You should be fine to disregard the issue until or if more notable issues arise without any affect on your typical usage.

As an aside, if you've changed the rail, have you changed only that component, or have you also replaced the circuit board with the upper ribbon cable attached? If you've replaced both and your prior issue wasn't to do with any of the rail inputs, you may be able to swap that board to resolve the issue. It is minor however, so there's no need to worry much.