venting propane stove

We are trying to install a used Comfort Glow 50K btu propane space heater for my mom. It has a 4 inch vent in the rear and will be vented into a s.s. 8 inch lined masonry chimney that we are planning on lining with 4 inch flex liner. Problem is that there will only be about 4ft of vertical pipe if we use two 90 degree elbows and the existing opening from a wood stove. Could we use two 45 degree elbows and then connect the pipe so that it enters the main chimney at an angle rather than up and over? Is there any other way to compensate for lack height for the vertical pipe without knocking another hole in the chimney. Thanks. Julie

The instructions call for minimum of 5 ft vertical. Just to clarify...this means that it has to be vertical and that 5ft at a 45 degree angle into the chimney will not give me the draft that I need? Do to the location of the existing chimney the stove will need to sit along side of rather than with its back the chimney, Could my other option be an elbow at the vent opening and straight over to the chimney into a new opening and connecting directly into my new 4 inch flex liner? The instructions are here http://dl.owneriq.net/3/3f704a53-a2ea-d814-5910-bb7e1287ccc2.pdf
They show a similar hook up. Just want to make sure I'm on the right track. Thanks Julie