Good point, this module has a 5x20mm 20A/220VAC Fast blow fuse at the output way. Might have been better to have it at the input way, but it's better than nothing.
Notice that terminals 1 and 4 are short circuited on the board (see picture). You might then prefer to connect the same type N (neutral) or L (live) on terminals 1 and 4.
Notice too that the main 220VAC (L or N depending of your terminal connection) is present at the potentiometer (not its body of course) and board is quite near of the metallic enclosure. Isolation would be a plus especially when it's used at high power.
Knob is a bit to big, so it's difficult to well see the graduation scale. They have put the graduation sticker on the blue plastic protection, so you have to remove it and put it back once you have take this blue plastic protection off.
Anyways, this is a nice little 220VAC power voltage regulator. I did not check it up to 4000W, but was working with a 100W motor without problems.

Good device. Tried that on home appliances with AC motors and all can be regulated well. It has no speed control and on low RPM you not getting enough torque, but for money you pay it's great thing to work with.
Tried on hand drill and washing machine motor, both work ok. I can hear some harmonics on low RPM which means that output is not a filtered clear sign ware, but device does it's job.
Design of terminals is very clever as screw 2 and 3 are both phase wires. It means chance to have short circuit between two screws (water, metal chips, loosen wire or its parts, dust) is much lower. It's small thing which could be important for long term use.

As you can see from picture there is no dedicated ground connection. Temporary I've connected it to available screw on the left of 4 screws used for power in and out. Do not thing that you will be lucky and without ground and some DIY equipment without double insulation. Make a ground connection and use RCB breaker feed power connection, otherwise you may get killed in case of internal electronics failure or dust or water causing phase to tough casing -> then you. RCB will power off in such situation and in most cases you will get only 30ma (industrial set to 200ma). Make sure to use device one you know about safety, double insulation of toolds and appliances, what is RCB breaker, what is Phase and what is Neutral (in US there are two phase wires).
Overall I recommend this device as it sustained ~400-500 watt load for 5 minutes without smoke or burning and appear to robust.