I have 120,00 miles on my 2007 outback LL bean. I would like to travel to  the arizona desert, My mechanic said it may need wheel bearings. Should i replace all 4?

the rears go 1rst ps followed by ds

What indication is there that there are bad wheel bearings? Do you hear noise, especially when turning? When road wheel is off the ground, moving road wheel up and down, and right to left, is there any "loose play?"

 

A bad wheel bearing gives you plenty of notice that it is going bad, and lets you drive for a long time before needing replacement. Wheel bearings are replaced when they go bad, not as a precaution against future failure.

Unless they are making noise, leave them alone.

 

"If it aint broke, don't fix it."

Last 3 I did were all 135000kms to 165000kms

If it's making grinding or 'wump wump' noises, replace it as it can damage the hub if the bearing goes too far.