Must I replace all 4 tires on a AWD Honda Element

Only if they are all worn down to a dangerous point. If you don't replace all four, put the new tires on the front and the two old ones with the most tread remaining on the rear. Handy way to save a good bit of money when buying tires. Also, if the included spare is a compact or "donut" spare, replace it with one of your old tires.

It is best to replace all 4 if you can. If there are budget constraints then put the two best on front and two new on the rear. This rule applies to both FWD & RWD vehicles. The reason for this is that most vehicles that lose control in loss of traction situations have the rear wheels break free and slide around to the front and consequently steering control is lost. If the best tires are on the back, then the rear of the car stays where you want it and the front wheels can still steer the car where you want it to go. In a FWD car you will not drive faster than the traction that your drive wheels will allow.

Im glad you capitalized the AWD part of your element. you wouldnt have to if you had a 2wd. AWD elements, crv's, and crosstours must have all 4 tires replaced because of the unique dual pump all wheel drive system. you can damage the rear diff. if u dont because thats were the heart of the awd system is. But if your tires only vary a few 32nds in depth then you'll most likely be ok.