You can still find 3-band kits online. 4-band is a challenge. I remember that back in the days it was common to see 3-band units, but no 4-band units. My own attempts at building a 4-band unit resultet in a heap of electronic garbage
.
A Google search turned up this
well documented 4-band design. Unfortunately it is completely in German only. The full schematic is on
this sub-site. The filters are fixed, not variable, but the filter characteristics can be set by design
as described here.
I tried a
Google translation of the website. The result is not bad. Probably sufficient for you to build this unit.
A note on the side: When dealing with mains you risk a life-threatening electric shock. Take all the necessary precautions and observe your local regulations. It is entirely up to you to fulfill all necessary safety standards. When in doubt, get help from a professional person.
Alternatively think about using LEDs (LED lamps) and a 12 V or similar low voltage system only. This will require a beefy power supply, but the risks are considerably lower.