Hi Jackschicks!
Seems to me if you've had chickens for 4 years without any problems, then you are doing it right. BYC is full of poultry horror stories. It's where we come to vent our complaints and look for help. By comparison, not many people come to post about another day with healthy chickens, but that is the norm for most I think.
Chicks coming from a reputable hatchery will not bring disease to your flock. Bringing in chicks or older birds from any other source comes with some risk. The wild birds and mosquitoes pass the diseases around too.
There is a lot of info out there about vaccines, some are worth it and some not, depending on your area and how many birds you have. There's probably a BYC thread for your area where people could give you the scoop, or check with a local university ag extension. A poultry vet here in Calif said that the vaccine for Mareks is not really all that effective if you do your own. The hatcheries do it before the chicks hatch, and even then it doesn't completely prevent them getting the virus, but can lessen the severity, especially preventing tumors.
If you do have any trouble, then this is the place, and I can tell you from personal experience, it's a huge relief to have the collective experience of so many helpful people when you need it.
Oh, and we love pictures!
Especially baby chick pictures!