My second plane was a 4 star 40. It makes a great second plane. Built stock, it will really float on the landings. Now, in my second season with the 4*, I have modified the plane by going from an OS .46LA to an OS .50SX, and removing one bay from each wing, shortenting the wingspan by about 4 - 5 inches. This adds lots of power, makes rolls much quicker, and landings are still very gentle. It's almost like gettiing a third plane for the cost of an engine. If you ever build a 4* I would recommend adding extra hinges one bay in from the tip. This will allow you to make the clipped wing mod easily.

As for when you are ready to move to a second plane, the advice given by others is good. Being able to recognize when you are getting into trouble, and getting out of it before stalling or loss of control is the biggest. I spent a lot of time putting my trainer into unusual attitudes and correcting them. I spend a lot of time practicing stalls and flying near stall speed.

Scott