I used to have a 4HP, this was too big, weighted stern down too much,
unecessary power.
Only problem is that the mainsheet can get caught on the tilted up motor
during a gybe (when the wind is behind you and the sail goes right over from
one side of the boat to the other) which can be dangerous in strong winds.
I've largely eliminated this problem by removing the rear traveller
arrangement and using a simple rope bridle instead
http://www.wayfarer-international.org/WIT/race.related/RiggingTips/UncleAl/Bridle/98Bridle.htm ,
attached slightly ahead of where the traveller is, and moving the attachment
point for the mainsheet pully at the stern end of the boom slightly forward.
Many sailors have given up on the rear traveller in boats like ours, more
trouble than its worth, you can get the same said control using the
mainsheet and boom vang.
For information on this and much more, see the Canadian Wayfarer Association
website technical section
http://www.wayfarer-international.org/WIT/WITindex1.html .
This is a very good resource, since the Mistral is almost identical to the
Wayfarer.
Good luck, the Mistral is a nice boat, ideal to learn on.
Bill Grant