horsepower gains for 2004 ranger

I have a 2004 Ranger Edge 4.0 liter. I have 500 dollars to spend for engine modifactions but i dont know what to get that would be the best for my truck and would give it the most horsepower gains. My truck is a lease vehicle so i can not do anything that will be too drastic such as exhaust just things i can take off after a year. I would like to buy a new chip although i could not do that untill a tuner comes out because my newer truck has something that wont let me get a chip with out a tuner. PLEASE HELP me wit suggestions.
I currently have blue neons throughtout the interior and underbody, halogen lights, 2 10 inch subwoofers and 1 6 1/2 inch subwoofer, blue suede interior, but i am now looking for items to increase my horsepower.thanks

its not yours to mess with. you shouldnt waste money on something that isn't yours unless you plan on buying it after lease.

i plan on buying it after it is over, although just incase i dont wanna do anything that changes it afterwards.