@Craig_A4
This setup is possible and should work.
The only caveat is not every brand of powerline adapters is compatible with Deco mesh Ethernet backhaul. Worst case, you may also need to replace powerline adapters with compatible brand.
Because Deco uses standard backhaul protocols, it is quite likely if your powerline adapters are not compatible with Deco, they won't be compatible with any other WiFi mesh systems and will need to be replaced anyway.
When you get your Deco M4, if you are not sure they work with your powerline adapters - ask here and I'll help to figure.
TP-Link does not sell M4 in single units. If you confident you will need four - buy two 2-unit sets of M4.
An alternative would be to buy 3-unit set of M4 as you plan, and later add Deco M5 to it. TP-Link sells Deco M5 in single units, M5 can be added to the Deco M4 mesh just fine.
Deco M5 form factor is like oversized smoke detector. It can be easily wall mounted or ceiling mounted with third party mount holder. If you find it more convenient to have Deco unit wall or ceiling mounted in the garage, consider M5. I can give you more info on different types of mount holders, if you are interested.
Check operational temperature range of Deco unit you plan for garage. For example, at my garage in the winter temperature drops below freezing, which is oustide of operational range for M4 and M5. You may need an outdoor Deco unit if your garage is not insulated and either too cold in the winter or too hot in the summer.