I have accepted an externship that has no compensation. Although I wish I had money from whoever wants to give it to me - I personally felt a relief about taking a 4th year without compensation.
Every site varies. I have heard that some sites that give compensation then infer that the student is (at times) essentially an "audio-bitch" and/or tech. I have heard that the student can seem more like a staff member and must take on the grunt work, especially when a staff member quits, and a replacement is not hired. Grunt work is expected, yes, but not when it is sacrificing the well-rounded experience you, as a student, should be getting.
For example, I know someone who received little training in cochlear implants, which is the reason she took a specific 4th year. Because a staff member left, she was the person to fill in (instead of hiring a new employee). Because of that, she did not touch a hearing aid or cochlear implant until the end of her 4th year and ended up contacting a solely CI site to try to get some hands-on experience. I'm not saying that the fact that she was paid was why she was in that situation, but sometimes if you receive a stipend/salary from a clinic, your needs as a student may become less important than their needs as a clinic.
I am happy that my future 4th year site views me as a student who will practice and explore all of the areas it has to offer with appropriate supervision. Though I must say, that I was lucky enough to interview in a location where I can live with family for free.
In the end, once I graduate I will (knock on wood) have a job and pay back my loans. I know that the loans I am accruing from grad school have a definite occupation/salary in the end. That's when I'll worry about money. Until then, I'm just going to relax and enjoy the ride.