@DingoDave 

While I was unable to locate an official replacement part for your M4, I did find an aftermarket alternative. Although I am uncertain about the exact power requirements of the M4, customer reviews suggest that many users have had positive experiences with this aftermarket replacement.

https://tinyurl.com/3nm3wpr7

  @DingoDave 

Hello Dave,  We do not sell them but you may be able to get them through a part request.  You just need to contact support via support.USA@tp-link.com.  you will need to provide the following information 

Name

Email

Shipping Address (no PO Box)

Model 

Hardware Version

Part needed

Reason for needing the part.

If we have that particular part in stock we can ship it to you.  Please note this only applies to the US.  We do not have access to the policies for other regions.