Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?
At the same size and rpm, a 4-bladed prop will require twice the horsepower to drive, approximately. 

Due to blade interference, it will always generate less than twice the thrust. So lbs thrust per hp will generally slightly decrease, as long as the propeller is in its optimal tip airspeed band.

If the propeller was overly fast initially, slowing it down and going with more blades to compensate can result in small thrust/hp gains (+X percent, not X times). This will generally apply to aircraft originally designed with much less power and later modified.

There are a few legitimate cases where design size is less than the aerodynamically optimal size. One is clearance to the ground or between the props (in quads). Another is turbofans: the drag of the nacelle overcomes the benefits of lower disc loading. Even for airliners there's a push for wider fans, but the "open fan" idea (basically a turboprop) was scaled down to geared fans and the Ultrafan for noise reasons.

In helicopters, at some point, the blades become so long and so far apart that adding more blades beats the weight and bulk of longer blades. The more blades, the worse, so going up to 4 is not such a big deal as doubling it again to 8. For small aircraft with low power/weight ratios, where 2 blades fit easily, that's generally the best configuration.