I've read warnings about when using drugs like Viagra that you should seek medical attention in the rare case of erections lasting 4 or more hours. My question is, is there any danger in erections lasting that long or longer, but just as a result of natural, prolonged stimulation and sexual exitement (no drug involved)?

In theory, yes, erections can last for longer than that, but it would be rare. If you have an erection for ANY reason, seek medical attention