Thought I would post an update on this one as I think I've solved my problem.
I was in Grain and Grape the other day (LHBS) and I struck up a conversation with one of the staff there. They said I should buy a reducer barb (10mm ID to 6mm ID) and force 4mm tubing onto it. I was sceptical that it would fit but they assured me with some hot water it could be done. I decided to give it a go.
Turns out it can be done but it's freakin' hard! I munted a lot of tubing before I finally got it right. In the end I heated the end of the tubing in boiling water for a minute, then forced in the blunt end of a 4mm drill bit. Once cooled I removed it, put it in boiling water again and then inserted the blunt end of the 4.5mm drill bit. I repeated this until I got to the 6mm drill bit and then forced the hose barb in. So it is possible but it's definitely not easy.
Next I wanted to see how fast it drained. This whole thing began when I started doing experiments with mash tun draining speeds and found it has a significant impact on efficiency.
I drained 5 litres through the 4mm ID tubing in 4 mins and 56 seconds. So for simplicity let's call it a litre a minute.
Coincidently I just purchased an SS Brewtech mash tun the other day and in the instructions there is a bit about drainage speeds. It reads:
The InfuSsion Mash Tun is designed to operate best with a run-off velocity of around 1 inch/min. Which is equal to about 0.67 gallons per minute of run off. You can increase your efficiency by slowing down the run off. Run off rates at less than 0.5 inches/minute are generally considered to be slower than necessary. Run off rates greater than 1.5 inches/minute will be less efficient and run the risk of a stuck sparge.
So converting this to metric optimal is 0.67 gallons per min or 2.53 litres per minute.
The slowest recommended is half this which is 1.26 litres per minute.
I'm draining slightly slower than this which I'm fine with. I'll drain a typical 30 litres mash and mash out volume in 30 mins which I think is reasonable.
The other advantage of using this method is that I can make another one and drain from my HLT to the mash tun to fly sparge and my flow in should be the same as my flow out.
The secondary purpose of going down to smaller tubing was for draining the boil kettle to the fermenter. I've found the slower I drain the weaker the vacuum sucking out wort is and that results in less kettle trub being sucked out. I suspect 1 litre per minute is a little strong for what I'm after but I'll see. I'm also thinking of getting the SS Brewtech boil kettle with the trub dam and maybe the two combined will keep most of the trub out.
Anyway, thanks for all your advice everyone.