While you might not be able to take yourself and your passengers on long-distance interstate stretches in your Polaris RZR4 without ending up like the Griswolds from National Lampoon's Vacation, you can definitely get some decent road-time done in an RZR with the appropriate street legal accessories. For safety reasons, there are several common street legal accessories for the Polaris RZR4 that most governments deem as requirements for being road legal. Doors and DOT approved windshields are mandatory in some areas, while license plates with license plate lights are required in others. Most street legal accessories that are prescribed and enforced by law are done so for a reason. So even if a particular RZR4 street legal part isn't required by law in your area, you may still find them useful nonetheless. Things like turn signal kits for UTVs are generally included in state or county traffic codes, but they are also quite useful for rides on crowded trails or when driving in a convoy. Especially for those who don't use radio communication systems, turn signals are the next best way to organize a group ride.
Like blinkers and light signals, mirrors are another example of street legal accessories that, even if not required by law, are good to have. After all, no one wants to kink their neck looking backwards after a long day on the bumpy trail. Mount the side-view version on the A pillars of your RZR4, or attach a rear-view mirror just below your aftermarket roof. DOT approved street legal accessories might also need to be installed for your RZR4 to pass your state's safety inspection. DOT tires and belts are often mandatory, and you'll need a place to mount your inspection / registration sticker as well. If you live in a sketchy area, make sure to safely secure your street legal Polaris RZR4, as it will draw a lot of attention. And for any street legal accessories, parts, or replacements for your four door Polaris RZR, know that Everything Polaris RZR has your back!