Recently I found out that the company a friend of mine co-owns uses 4D, which I've never heard of before. They swear by it, but they're non-technical and what they say about it sounds like memorized marketing blurb. Unfortunately the 4D website also seems devoid of any actual information and is filled with words like "comprehensive", "solution", "platform" and "integrated" instead.
Since that thing is rather expensive and uses a custom language that I don't have much inclination to learn just for one project, I'm cautious about it and I'm wondering if anyone had any experience with it? Would you recommend it? What is it good for? What competitive advantage would I gain by learning it as a programmer, or using it as a company?
4D has been around for a long time (~25 years), so it's much older than e.g. MySQL. Think of it as a professional version of Microsoft Access: It has its own Pascal-inspired host language, its own relational database engine, a very mature IDE for rapid GUI development and a custom runtime which allows for true "write once, run anywhere" (anywhere being Mac OS (X) and Windows, that is). Nowadays, it also understands SQL, there's a server version and even an integrated web server. It's fairly powerful, so the comparison to Access probably does not do it justice.
Today, I believe it's mostly used for legacy apps which are as old as 4D is. I don't think I would learn it again today, much less start new projects with it, since you can get the same functionality and then some by stacking up open source components.