Is this possible?

Sounds like you're making progress!  

Can you post your Shield activate-source macro?  That's where I would guess the problem lies.

Kyle

macro|AV|{{active_tv_{{clientname}}}}~Remote~INPUT HDMI 4!activate-source.shieldtv-shield - bedroom

I've attached the full macro as well just in case
 

I'm not sure if this is the issue, but it looks like your device names have spaces in them.  I think they're supposed to be one word.  My Shield is named BasementShield, my TV is TheatreTV, etc.  Having a space might be confusing things.

Also, what kind of TV do you have?  The command has spaces in it, which also seems odd.  I have a Sony TV, and an input command looks like "AV|7~Remote~HDMI1" with no spaces.

I attached my Shield activation macro below.  You can see where I have the display and AVR turned on (displayon.{{activeroom_{{clientname}}}}) and then set the inputs for my AVR and TV.  I first set variables to where they need to be (HDMI3 for the TV, MPlay for the AVR) and then the commands are executed.  Your macro already has the command to change the input on your TV, but the variable (SetVariable|{{activeroom_{{clientname}}}}_display_input) needs to be uncommented and set up like this for the Shield:  SetVariable|{{activeroom_{{clientname}}}}_display_input~HDMI4!

Kyle

Here's my macro.

Sorry for another post, but an easy way to get commands correct is to use the command builder on the driver page. 

For instance, to build a command to change the TV to HDMI 4, copy it somewhere, and then replace your AV ID (AV|X, where X is your AV ID) with:  {{active_tv_{{clientname}}}}

For me, this is what I get from the command builder:  AV|7~Remote~HDMI4

I copy this and replace "7," so it looks like this:  AV|{{active_tv_{{clientname}}}}~Remote~HDMI4

Im out and about for a few hours but there’s plenty for me to get my teeth into later on

And you sir are a legend