That'll work for about 0.02 of a second. 3-4A @ 50V would require more than 16-22A @ 9V which no little 9V battery, even in it's wildest dreams, can ever hope to supply. Are you actually trying to build this or just SIM it?
That simple boost converter might be just the thing to convert the voltage--the amperage I don't think you're going to be happy with, though. 3 or 4 amps from a little 9V battery will kill the battery very quickly, even if the boost converter can maintain 50V with that kind of load on it.
What are you planning to use this thing for? If you say what you're trying to accomplish, instead of how you're trying to accomplish it, we can often come up with an alternate solution which may be simpler or work better.
I was hoping to build this...i want to power up a motor that i have and also do other duties. I thought why not designing sumthing that could run of at least a 9v battery.what would be achievable?