Not a big fan of emulation, so I will skip that part of the question and instead talk about the hardware/build quality pros/cons:
+ a better quality build, looks durable enough to survive a fall on the floor
+ usb port for plugging external devices (like a gamepad)
- price, at least twice as expensive as a dingoo
- analog stick (instead of a D-PAD), the analog is also "clickable" and can be used as a button if pressed at the center. This solution might lead to analog damage if abused by a 4 year old (I have experience with analogs in gamepads breaking from overusing the clickable part)
- no TV-out cable included, requires buying it as a separate accessory
- EXT port used for charging/tv-out instead of a standard miniUSB approach (lose the cable and you can't charge your Caanoo anymore)
- EXT connector part of the charging cable has those "latch" things that keep it in. They need some strength to put them in and they look like they could be damaged if the cable was pulled out by force
- heavy weight, might be too much for a 4 year old
+ TV out cable that comes with the device
- made of a quite fragile plastic
- easy to break the LCD by dropping it on the ground or sitting on it (buying a rubber protective case is advised)
- shoulder buttons (L/R) are fragile and break for some people
- no handstrap
Both devices will require a grownup guidance with installing games/emulators, although in both cases it's fairly easy. Flashing the alternative firmware (Dingux/OpenDingux) for the Dingoo is recommended.
As for the Letcool, I never had one, so I can't say anything.