Ok, you can try two things:
Download this file and install it (with the update), afterwhich, in the main Dash 8 Q-300 folder, copy the air folder, to the Dreamwings Q400 folder. (DHC8Q3.air)
Open the Fanda aircraft.cfg
title=Dash8-Q300 V1.004b. Tyrolean Air livery.
sim=DHC8Q3 <COPY THIS
ui_variation=Dash8 - 300. Tyrolean Air livery.
description=Dash8 - 300. Tyrolean Air livery. copyright by Oleksiy Frolov 2002. Version 1.004b
Copy the above line and put it overwrite, the Dreamwings Dash 8 Q400 Simline (in every paint)
don't forget to back up.
As Fanda's systems are very complicated, it might not work without the panel.
Or, if you want the panel with the Q400 (if you like detailed systems)
follow this, The FDE isn't completely Fanda's, however Aparently its a 'good mix'.
* If you like to use default, basic panel systems, try replacing it with the Fanda FDE.
* If you want to learn to 'fly' the Dash 8, with nearly ALL systems, (freeware) [Q300, but quite close to the 400 I'd say, even though the 400 has fully glass displays], Try the tutorial.