Well I changed my oil today, and afterward I started it up.. checked the plug and filter for leaks... shut it off and it started hissing. And the A/C pipes were cold, but the AC is not on. What gives?

 

The AC hisses when shut off as the high and low pressure sides equalize in pressure.

On your 2004, the AC compressor is enabled in the folowing modes:

Defrost
Floor/Defrost
Floor/Panel
Floor only
AC
Max AC

The only modes where it is not enabled are

OFF
Panel only (vent)

 

Bringing this thread back alive...my '98 is hissing while on.
I can tell the compressor is on and then it clicks off and hisses inside (cold air still blowing throughout all of this), then clicks back on, then clicks off and hisses inside, etc.
I had a buddy charge it on sunday, its tuesday and I think I'm already running low on freon again. We sniffed it and couldnt find a leak, so we assumed on sunday it was leaking in the evaporator coil. Any thoughts on this? Thanks

 

When i saw the post my first thought was the click clack problem. It is a known issue. There is a TSB on it. I never bothered to fix mine on my 1995 because I read somewhere it is not harmful, just noisy.

Here is a link to the known issue for my year. You can find yours by doing a search.
Automotive Recalls and Technical Service Bulletins for 1995 Ford Truck Ranger 4WD V6-182 3.0L. Provided by ALLDATA

01-21-2 OCT 01 A/C Clutch - Loud Engagement Noise

But they want $$ if you want the info to fix. Someone here may know what to do but Ford seems to have known about the issue and what to do.
You can also check with your dealer, if they will give you the info.

EDIT: Of course the hiss, I don't know. The clutch click clacking might be this, in my totally unprofessional opinion.