@cbrand30 

Let's start with aligning expectations. In my tests with Satellite Deco M5 (which is equivalent specs to M4) the top WiFi speed I ever get on smartphone connected to it was 225 Mbps. This is after very many tries, with idle Deco mesh network and very little interference from other WiFi network and household appliances. 

My Satellite M5 is placed in such way as to get good WiFi backhaul signal from Main Deco. To see if your M4 is placed well in regard to backhaul connection, open M4 node page in Deco app and check signal source. This is my Satellite M5:

If you have 2-3 blue bars on connection, it is good place for Satellite Deco. If you see worse signal quality, you'll need to move Satellite Deco closer to one it is connected to (usually Main Deco).

You have decent placement of Satellite M4s, but there is always room for improvement. These will be my suggestions, and again, check signal source on Satellite M4 before and after you move it to a different place. That'll tell you if new place is better or worse when it comes to backhaul link between Satellite Deco and Main Deco.

Unit #1, Main Deco: If you can move it to a different wall of the same room, please do. I marked suggested area for main Deco by red square on floor plan. See updated floor plan at the end of this post. If you can't move Main Deco, leave it as is.

Unit #2: There seem to be a number of obstacles between it and Main Deco, such as walls and other objects. Consider moving this unit to the area marked by red square on floor plan. If you have multiple electrical outlets in this area, try Satellite M4 at each of them to see which is the best spot, based on Signal Source quality reported for that unit in Deco app.

Unit #3: I suggest moving this unit to the middle of the first floor, and place it somewhere in the area marked by red square. That might not be necessary if you can move Unit #1 Main Deco as I suggested, but you can still try and see which place is better for Unit #3.

I can't promise substantial WiFi improvements, and if you start getting 200Mbps or more from device that is connected to Satellite Deco M4, call it a day. 

@Alexandre. 
 

thank you for your detailed response and for marking up the floor plans for me. I'll give this a go and will post the results!

I've only just got round to giving this another go as I've been away all week. Unfortunately im getting almost identical results as before. It doesn't seem to matter where I put the decos, the speed still drops right down. I really don't know what else to do apart from send them back?