Why can't you mix rechargeable and alkaline batteries? If item takes 4 batts, are 2 alk and 2 rechargeable ok?

Ideally you want all of the batteries in a set to have very similar discharge rates. If the rate at which they give up their charge is quite different, the weaker batteries will draw charge from the stronger ones instead of using it to work the actual circuit. Not a good thing for either type of battery (what happens to a non-rechargeable battery when you try to recharge it? what happens to a rechargeable battery when you try to charge it using the wrong polarity?). The best thing is to have all the batteries in a set at least of the same type, and preferably having the same level of charge.