my 89 brat stuck in 4wd , saw a post saying flex the car. what is flex?

Flex is the wrong term commonly used by “low educated” 4wders, they actually need to use “articulation”... 

What you need to do is jack up a wheel with the others chocked well to stop the car moving off the jack. The lifted wheel will spin at some point. You should be able to release 4wd. 

Or go for a drive on a dirt road/with one set of wheels on the dirt, while moving apply some pressure on the 4wd lever, it’ll slip out. 

Lastly, while your brat might be a ‘91 model, you need to post in the EA platform section  

Cheers 

Bennie

Sometimes my brat feels like I’m stuck when parked. It shifts out of 4low easier when I am slowly moving let off the gas and shift clutch in of course. Or reverse and shift out clutch in. With out more information hard to tell. And el Freddo answered you original question perfectly.

So I will adjust my kid to know that it works.https://www.allicdata.com/blog

Not sure if the OP knows or if it was part of the issue here, but some rules for using Part time 4wd on older subaru;s (and all true 4wds)

1) NEVER engage 4wd on pavement, concrete, etc.  Loose pack dirt, gravel, mud, or snow only.

2)ALWAYS have 4 matched tires.  

3) If you do break rule 1 and 2 together, then you are really in a pickle.  Different size tires rotate different amounts for the same distance.  If you drive on hard surface while in 4wd you WILL need to lift a tire off the ground to disengage 4wd, or drive slowly, straight to the nearest patch of loose gravel.  Sometime simply getting both tires on one side off the road on the shoulder and driving straight while pushing lever down (or hitting button) to disengage.