Progress is a niche product rather than a mass market product like Java or any of MS' stuff. Further Progress is sold indirectly -- almost all Progress licenses are associated with an application that was created and sold by a partner. Even most "direct" licenses were initially sold by partners. The customers become direct customers either because they scale beyond what the partner can support or the partner has become defunct but the application lives on.
And "Progress" is a very difficult term to Google successfully...
There are positives and negatives to all of that.
On the negative side there is "Progress who?" Many people, even in IT, have no idea what Progress is. And if they do have an idea it is often negative -- usually out of ignorance or because they have generally been burned by "off brand" solutions in the past.
From a career advancement perspective there is the problem that Progress-related jobs aren't usually categorized as "Progress". They are generally cast as "MFG/Pro" or "Trend" or "Epicor" or any of several thousand other partner applications. So the job market is very fragmented.
But there are some bright sides to that -- if you have good skills you can become very, very valuable to your employer. It is unlikely that HR is being flooded with resumes that have "N years of experience with XYZ application and Progress 4gl" all jockeying for your job.
This should put anyone with good skills in a good negotiating position. (But don't get too cocky -- the truth is that 4gl coding is easy to pick up and you can be replaced if your employer is sufficiently motivated...) If you play your cards well and combine good coding skills with excellent understanding of the business problems being addressed than you can do very well with Progress. Of course that's true of any technology... but IMHO because Progress is a niche you have more leverage and can therefore do better -- you aren't competing with nearly so many people who are willing to bid lower.
Of course you'll have to endure the slings and arrows of the "Progress isn't Java" or "Progress isn't Oracle" or "Progress isn't XYZZY" crowd. Just smile and tell them they're right, it isn't those other things. It is Progress and it happens to be supporting the business very well. When they blather on about how such and such a feature is missing and therefore Progress isn't good enough, smile and pat them on the head like a puppy dog that will pee on the floor if it doesn't get your attention. Then go back to solving the problems that you are paid to solve.
<-- That's me --> puzzled as always