4D is great for general super low wind flying (few will go lower - it will fly well in nearly nothing), but I think you need to have some expertise in order to trick it well - its no yank and spank sort of kite, but requires some finesse in the older-school style. I like it best in 0-3mph for tricks, but I could see just flying around up to 7 or so (I don't understand the 12 mph rating on this kite). It is also fragile in terms of connectors, but tough otherwise - buy some of the aluminum ferrules it uses at the same time you buy the kite - you will need them (at the centerT and also to connect the LEs). It has become my favorite near-no-wind (<1mph) kite.
The Shadow needs a bit more more wind than the 4D (I'd say 0.75 mph more), but it tricks more easily and handles rougher inputs more readily. Also, it is much stronger and a stiffer (flys much higher without concern) compared to the 4D although both are flexible frame kites.
Both of these kites I'd consider great low wind flyers (lots of drive), but more intermediate to advanced in terms of tricking them - they are not too forgiving of too much or the wrong inputs. Either would be a great kite for someone who simply wants to keep flying with ease when the wind is <3 mph. You can literally just stand there, even walking forward if you wish.
The Zephyr needs far more wind than either of these imo (say 3-4), but if you have the wind, its lots of fun.
They will all make your adrenaline feel like a lead ballon in 4mph of wind so you can't make a bad choice. Good luck!