EDIT: If you are running the woofers straight and the tweeters with only a cap, the same thing applies as the rising impedance of the woofer puts it high enough so as not to affect the overall impedance when you add the tweeter with it's cap.

What is the impedance of a tweeter?

Usually rated impedance is either 4 Ohm or 8 Ohm as you probably know. If you have a DMM, try measuring the DC resistance of the tweeter. Then rated impedance is about 1,2 times the DC resistance.

Can I use a 4 ohm woofer with an 8 ohm tweeter?

If you use a 4 ohm high frequency driver with an 8 ohm crossover, the crossover point will double. If you use a 4 ohm woofer with an 8 ohm crossover, the crossover point will halve. The result of using 4 ohm components with a 2-way 8 ohm crossover will leave a hole in the response.