What will happen if i put 2 stroke oil in a 4 stroke lawnmower engine & run it ???????????????

do you mean in the fuel or the crankcase...if its a normal two stroke mix in the fuel it will be ok the worst it can do is oil up the spark plug but it should be fine ive done it plenty of times before with no problems....if its in the crankcase you need to drain it and replace it with the right grade of oil...it should tell you what grade on the filler cap. i hope this helps you out