can i use 4,000 ohm headphones on my crystal radio?
 

So ive been using a standard 2,000 ohm high impedance earphone for my crystal radio. I went looking on ebay for headphones and found a pair that are rated at 4,000 ohms. What would i have to do to be able to use these? Would they just be more sensitive?

Actually the higher the impedance, the better. Less loading on the tuned circuit means higher sensitivity and 'Q' factor (selectivity). In the early days winding headphones to the highest impedance was an art in itself. It entailed putting on the most turns of the finest gauge wire, and women seemed best at it because of higher manual dexterity.
These days, crystal (piezoelectric) phones offer the highest impedance. Here's an example.. http://catalog.miniscience.com/catal.../earphone.html