$200 per window, PLUS the cost of the window is pretty average price.
One window is considered a small job – this means you will pay a little more.
If you have any curtains removed and furniture away from the window I would say an experienced carpenter can remove the old window and install the new in 1- 1+1/2 hrs., if it was measured correctly. The window should cost between $130 – $160 depending on the size. I assume it is a double hung window.
Should you wait until you have the money for all four windows, you should get a cheaper price.
Now remember minimum labor for only one window should be around $150 – $200 this is because it is almost not worth a good carpenter to go out and replace just one window.
Pella is a brand you can count on if you should have any problems. There are literally thousands of replacement window brands – glad to hear you choose Pella.