RE: 4-Balde props, pros and cons?
For model airplanes there very few pro's, except appearance. 3-blade, & even worse, 4-blade props, have extremely limited applications on models.
They are heavier & more costly than equivalent 2-blades. They are also less efficient -- primarily due to the necessity for reducing diameter, if the same blade pitch is to be retained, in order to keep the engine load within an acceptable range. Additionally, they have more blade-frontal-area and more wetted surface than an equivalent 2-blade, resulting in greater parasitic power losses.
Bubba has it mostly right about there being more left yaw resulting from the multi-blade set-up, although he is a bit off-base with the P-factor thingy. The greater airflow over the left side of the vertical tail is caused by the spiral slipstream from the prop. This pushes the tail to the right.
P-factor is something else altogether. It only occurs when the plane is at a positive angle of attack, as when a tail dragger is sitting with its tail-wheel on the ground. This causes the descending blades (right side) to be at a greater angle of attack than the ascending blades (left side) -- & results in greater thrust being developed on the right side of the aircraft -- thus pushing the nose to the left. When the tail rises & the AOA essentially reaches zero, the P-factor dissappears.
Where multi-blade props are usefull, is in their ability to transmit more power for a given diameter (disregarding lower efficiency). If a model has a prop clearance problem that would otherwise dictate a less powerfull engine because of limited prop size, a multi-blade prop can be the answer -- thus permitting a stronger engine to be used. However, the efficiency losses don't go away.