Alan Smallbone offered a lot of good information.
For the United States, the "where you can fly" question can't be easily answered at this point. It can be further complicated by whether you are flying as a hobbyist or for commercial benefit...and commercial benefit seems to encompass a wide range of activities. One example from the FAA is using a drone to see if your field is being uniformly watered. If you sell the crops, then it is a commercial benefit, but if it's your own garden, then you can do it as a hobbyist.
For recreational flyers, the FAA geographic limits are fairly easily determined. For example, if you fly within 5 statute miles of an airport, you must notify the airport (and air traffic control if they have one) of your intent, including flight location, maximum altitude, flight times. They have the right to disapprove. Of course, forget about flying in Washington, DC. Large stadiums are off limits. There may be temporary no-fly zones around such things as forest fires or hurricane impacted areas. You can find all of these both from maps published online by the FAA and through their free B4UFly app.
There are other restrictions, such as the operator must keep the craft within unaided visual line of sight. There's also the need to adhere to a safety code promulgated by national modeler organizations. They haven't said _which_ organization, yet. However if go look at the major ones...the names of which I can't remember right now...you'll see that they are common sense limits, such as don't fly over groups of people, etc. There's a max 400 foot AGL restriction. Well, it is modified by if your within 400 feet of a structure, your altitude is limited to 400 feet above the structure. And of course, you must always give way to any manned craft.
If you do a very quick search on YouTube, you'll see that these are violated daily.
Then there are state and local laws, and these are damned hard to find. For example, you cannot fly from a Denver, Colorado city park, but this is listed in a parks regulation document...along with hitting a golf ball. There are exceptions, for example you are allowed to fly in an RC park and hit golf balls in a driving range. It took me forever to find that particular restriction. And it doesn't address whether or not it's illegal to fly over a city park, only that you can't fly from the park.
Add to this various common law rights...is it a trespass if you fly over someone's house?...which have not been clearly adjudicated, and it gets a bit complicated.
Common sense and a bit of politeness will help.
As to the Phantom Pro 4 camera. It's a 20 mb Sony Exmor 1" sensor. The focal length is 8.8 mm, which would be a 24mm equivalent. I recall that it's a seven element design. It has a mechanical shutter for stills, ranging from 8 to 1/2000 second, with an electronic shutter from there to 1/8000 second. The aperture is from f/2.8 to f/11. Base ISO is 100, going to 12800. For exposure it has auto, shutter priority, aperture priority, and full manual modes. The lens itself is fairly decent with some softness in the corners. With the 3:2 (full frame) aspect ratio there's some series vignetting in the corners. The camera has auto focus (not continuous...tap to focus) and manual focus. It can supply JPEG and DNG (raw) images. LR can read the DNG, but there's no official Adobe lens or color profiles for this camera at this time, although Adobe and DxO do support the Phantom 4, which uses an entirely different camera.
Okay...all that being said...it's a point and shoot. While the dynamic range is actually pretty decent, there's luminance noise even at ISO 100 in the DNGs, particularly in the blue channel. Don't expect to be printing large from this camera, or at least not without a heck of a lot of post processing. I can print an acceptable 13X19" (12X18" image area), but my definition of "acceptable" probably wouldn't pass muster here.
As pointed out, if you want better IQ you're going to be spending a lot more money. DJI's step up would be the Inspire 2 with a ZenMuse X5S, which is going to set you back $3000 for the aircraft, $1400 for the camera and gimbal, and some additional money for a lens (there are compatible Olympus and Panasonic lenses). And you probably need another $1000 for the DJI Focus control, plus money for additional batteries, carrying case, and other incidentals. And this just gets you to a micro43 camera.
What it can give you is a different viewpoint. The world looks very different at 100 feet in the air. If you approach it with that in mind...and quit thinking that you're going to get the IQ of even an entry level DSLR, then you'll be happy.