1996 Legacy outback EJ22 engine. Low compression in #1&3 cylinders (75psi). #2&4 (175psi). No smoke, no oil in water, no water in oil. Tried Bars Gasket Sealer no joy. Plug wires are new as are plugs. No over heating. WTF is wrong? HELP!

Not a lot of details here to help give most useful response.

What technical issues having leading up to doing compression testing and has any recent work been done to motor ?

Initial assumption would be valve issues or valve timing on the poor compression reading head .

A good visual and bore scope might give some answers quickly, leak down test be useful too .

Car was running fine. One morning I was heading to work. 2 blocks from home after making a left turn it just started running on 2 or 3 cylinders. No noise proceeded this or any hint that this might happen. Checked plug wires, no problem there, plenty of spark at plugs. Cleaned air flow sensor just in case, no change. Went to my mechanic and he ran compression test and found the low cylinders.  Idles really rough at about 500 rpm tries to quit, used to idle smooth at 800 rpm. Up at 2500 to 3000 rpm it runs with plenty of power. I am living in an apartment complex so there is no way to put the car up to the point where the engine and transmission can be dropped to do heavy duty work it. Can't afford to have a shop do that work, only paid $400.00 for car. Estimates run from $1500 to $3500. Love the car but ain't got that kind of $$$$.

Mmm, Needs a good visual and leak down test .
If your mechanics compression results correct surprised runs very good at all, if you using no oil/water and compression truly that low on one side then it more than likely valve/timing issue or cylinder to cylinder gasket failure perhaps (has timing belt jumped/gone slack).
Such a sudden transition from running okay to failure is not norm for engine internal failure but could be for cam belt/timing issue .
Don't overlook basics until compression/leakdown double checked/verified with cause found .
Really needs competent investigation, heads can be pulled in frame pretty easily if clued up on it but if does need some level engine work  costs going mount up even if do some work yourself .