Well kph, Welcome to the forum. You are opening a can of worms but this is what I would suggest:
Correction: It is not a BIR Curry so you could go ahead and increase the ingredients linearly by X2 with a few possible exceptions depending on how hot and how wet you prefer your curries. There is enough in the original recipe for 2 or 3 as it is with 2 whole chicken breasts. I wouldn't use 4 tsp of chilli or 6 tomatoes. I'd go with 3 tsp chilli and probably only increase to 4 tomatoes. It depends on whether you are using a hot or a mild chilli powder as well. I don't think I'd be going with 10 green chilli's either but that would be up to you.
Another consideration is the other 3 people and how they like their curry. If I was cooking this for my family I wouldn't increase the chillis at all. I would add extra chicken etc but leave the chillis as they are.
As it is only for 4 people I would simply multiply the ingredients for 1 serve by double, cook 2 double serves and then combine them. You may need to reduce the amount of liquid (ie: base gravy) unless you like a wet curry. There is no real problem in linear scaling up to a double quantity and cooking a double quantity is easy enough to get a good result provided your pan is big enough.
Trying to cook a 4 serve quantity in a single pot will give you a different result. You could do it but it would involve reducing the amount of liquid and possibly adjusting the spice levels a bit (particularly Chilli). To do this I would probably modify to a more traditional Indian approach (or hybrid at least).
If you were to go to higher quantity like 8 or 10 then you start to get into the bulk cooking linear V non-linear scaling argument.