Before you try the 'Hardware Tool Emergency Boot Firmware Recovery' it would be helpful for you to try something.
First some background:
The MPLAB ICD 4, MPLAB PICkit 4, MPLAB PKoB 4, and MPLAB Snap ICD tool all use an ATSAME70 MCU.
21B for the MPLAB ICD4, and MPLAB PICkit 4, and ATSAME70N
21B for the MPLAB PKoB 4 and MPLAB Snap ICD.)
The ATSAME70 MCU has a ROM-based (fixed, not erasable) program called SAM-BA (SAM Boot Assistant).
SAM-BA will USB-enumerate as Virtual COM port.
It could be that the MPLAB PICKit 4's 'Emergency Recovery Button' was pressed (via the small hole near the USB connector) while being plugged into the USB port. If this happens the SAM-BA will be active and no lights will be active on your MPLAB PICkit 4 tool after a subsequent power cycle of the tool. This is because the 'ERASE' pin on the ATSAME70 MCU ultimately gets asserted which erases the tool's Application and Bootloader programs which are responsible for controlling the LEDs.
In any event, SAM-BA will USB-enumerate as a Virtual COM port so it will be easy to know if your MPLAB PICkit 4 is in SAM-BA mode -- you should see 'AT91' (or sometimes inadvertently as 'Bossa' if I remember correctly). Could you try this?
When you plug your MPLAB PICkit 4 into the PC (and no lights are lit as you have stated in your original post), in the USB list, do you see 'AT91' (or sometimes 'Bossa')? If you have a Windows machine you can look at Device Manager or use a free utility such as USB View.
If you don't see 'AT91' (or sometimes 'Bossa'), then try the 'Hardware Tool Emergency Boot Firmware Recovery'.
The full details of this is documented in the MPLAB PICkit 4 In-Circuit Debugger User's Guide on Microchip's web site: http://ww1.microchip.com/downloads/en/DeviceDoc/MPLAB_PICkit_4_In-Circuit_Debugger_User's_Guide_50002751E.pdf
What the 'Hardware Tool Emergency Boot Firmware Recovery' does (in a nutshell) is to utilize the ROM-based SAM-BA to re-program the Microchip custom flash-bassed Bootloader into the tool. Then, when you USB-unplug/re-plug the tool will execute the flash-resident Microchip custom Bootloader.
In this state with the flash-resident Microchip custom Bootloader running, MPLAB X IDE/IPE should now recognize the Microchip Bootloader and then will bootload the Application firmware into the tool and you should be up and running again.
I hope this helps.