The 9XL will pull lower amps. Depending on the C- racting of the 4s pack it may be the better choice. With that motor you can safely run the K40&42 carbon props, X440,442,X438,X437,X440-3 blade, Prather 215 and any other props around 37 to 43mm. I run the K40 on my 36 Cat and pull around 70 amps...