In theory it should be possible to fit a (nearly) A3 landscape drawing onto 2 sheets of portrait A4. The 'Posterize' print option matches the orientation of the paper to the drawing so that 4 sheets are used. 

To try to work round this I selected a custom page size for the drawing which was the same as A3 landscape (42cm wide and 29.7 tall). I then tried printing this as a poster.

It correctly printed the first sheet, but the second sheet, with the top right of the drawing, started printing the part of the drawing which is about 26.7cm from the left hand side instead of the 21cm which would represent the width of an A4 sheet. The result is that a vertical strip from the centre of the drawing, about 6cm wide is not printed at all.