A double story terrace house currently has 4 bedrooms & 2 bathrooms

There are 3 bedrooms at Level 2 and 1 bedroom at Level 1. There is 1 bathroom at Level 2 and another at Level 1.

is it possible me how much would it cost to add a toilet (ensuite) for one of the bedrooms at Level 2 and add a toilet (ensuite) for the bedroom at Level 1. It is possible to run a straight stack from Level 2 to Level 1 for these 2 bathrooms.

anyone here done this before? What is the ball park cost of this kind of project?

Really depends on the existing layout. Need to see the proposed layout. 

Sewer lines that carry the poo flows by gravity. For the new lvl1 toilet if it abutt the existing toilet still easy. If have a horizontal run that will cross ground beams is a major task. 

Same for level2 toilets. Need to see if crossing any beams, and if the lowered false ceiling is acceptable to the client

this is too expensive.

4000$ is the approximate cost for adding toilets.

4k is just too low. A reno of tiles for toilet floor, wall, water closet, sink, shower area etc false ceiling and lightings can reach 10k. This is for renovation alone. What more creating a new toilet.

Think you should budget about $10K per toilet. Depending on the layout and the size you want, it might involve other works as well.