Is my wiring for this 5m 2812B Led Strip correct?
Yes, that should will work. The power doesn't have to flow in the same direction as the data.

However, you may get better radio interference suppression (and generate less interference) if you try to run the data next to the ground - this would mean running the green wire from the end of strip 1 all the way back to the power supply, then out to the end of strip 2, instead of taking the shortcut.
I know this from experience, because I tried shortcutting the data lines in a bigger installation, and whenever a light was turned on or off in the next room, the next several LEDs after the shortcut would randomly change colour (until they received new data from the controller).
You might also be able to fix it by running an extra ground wire next to the data wire, but I don't know whether that works well. Someone else should know.

Make sure to get the LED strips in the right direction for the data flow - of course.

The current in the data wire is very small. You can use as small a wire as you want.

Since you're dividing the current 4 ways, the current in each power wire is 1/4 of the total current. When it's working. If there's an accidental short circuit, all the power from the power supply might go to one strip.

These LED strips only use their maximum power (about 0.25 W each) when they are displaying white at maximum brightness. If you aren't planning to have them display white at maximum brightness all the time, it's okay to plan the cooling based on the average power rather than the maximum power.

You can also get away with a smaller power supply if you promise not to set them all to maximum white at the same time. I used this idea to run an LED project off a USB charger (5V 2A).

The amount of heat in watts is the same as the amount of electrical power in watts. Not the maximum power, but the actual power at any given time. 216 * 0.25 is 54 watts maximum, and let's say you actually use half that on average, which is 27 watts. I don't think a box this size will have any problem dissipating 27 watts of heat, or even 54. I wouldn't worry about cooling.
Imagine putting a 60 watt lightbulb inside the box. I think it would get warm, but not hot.