Okay thanks for the suggestions - is it fine if I do the compression test when the engine is cold? I'm pretty sure I did all the others when it was cold.

 

Did the head bolts get reused when you changed the head and head gasket?

yes , cold is fine, pull all 4 spark plugs, do the compression test, then add a tablespoon of engine oil to each cylinder and do a wet compression test. Your oil rings might be gummed up.

 

I used brand-new head bolts when installing the head.

Here are the results of my compression test:
1: 135 dry, 145 wet
2: 145 dry, 155 wet
3: 145 dry, 160 wet
4: 135 dry, 137.5 wet

My dry compression numbers are higher than the last time I tested it, almost the same exact numbers I had 2 tests ago (about 2 years ago) except for cylinder 1 which went from 130 to 135.

All the spark plugs looked good with the same tan color for all 4.

Are these wet increases acceptable?

Also, how can I determine if it's transmission fluid in the coolant?

 

ATF has a red tinge. But, if you are using oil it either goes out the exhaust, on the floor or in the crankcase.
#4 kind of indicates valves, #1 is problematic. All are a bit low.

 

I think the oil rings are gummed ups common problem with long oil change interval’s. If engine oil is getting into the cooling system, then coolant will get into the oil pan, is your engine oil mysteriously higher on the dip stick?
if it is transmission fluid and coolant mixed that’s a problem with the radiator and transmission cooler

 

The color of the coolant is brown with no red in it. I haven't changed the transmission fluid before and it's at 72k miles, but not sure if it would really get dirty enough to make the coolant a brown color. My engine oil has been lower every time but it looks clean on the dipstick. I change my oil every 3000-4000 miles too. I noticed a little bit ago that the oil filler tube was coated with yellow-looking thick moisture. When I searched on the threads here, it said that it's usually due to not running the engine long enough. The most I drive in one trip is 10 miles usually. The filler tube doesn't have any of that now though.

 

Alright, I did a leak-down test. Cylinders 1 and 3 have 40% loss, cylinder 4 has 45% loss, and cylinder 2 has 35% loss. Every time the air was coming up from the timing chain area (did it with the valve cover off). No bubbles in the coolant, nothing coming from exhaust, nothing from valves. Can this be narrowed down to piston rings and cylinder walls, or is there something else that could be bad? How does the oil leaking into the coolant tie in with this? Also, as I was giving it more air pressure, the pistons sometimes moved down. I'm not sure if it moved back or forward. Should I be concerned with this at all?

 

Water pump?

 

Did you check for leakage into an adjacent cylinder?

That's a lot of leakage. You've listed cylinder 3 twice, and no cylinder 2. Regardless, it looks like they all have worn rings, and #4 has an additional problem, maybe a burnt exhaust valve. It can be very difficult to hear air leaking out of the exhaust tip. The sound has to get through 1 or 2 cats, muffler, resonator, and a bunch of piping.

Rotating the crankshaft backwards could potentially cause a problem with the timing chain.

 

I suggest either a Seafoam treatment or a quart of Marvel Mystery Oil with 4 quarts of 5w30 and a fresh oil filter. These might loosen up stuck rings the MMO was my go to when I bought an older car , it worked.
I would drive the car 3,000 to 4,000 miles then change the oil and repeat the MMO, then check compression again, sometimes an improvement most times it cured any blow by oil burning.
You can try this, or find a low mileage engine at a wrecking yard or have your engine rebuilt.