I have those sort of connectors for solar input into the Ecoflow battery that I have. I admit it is the only place I have seen them. Male & Female connectors are readily available from Battery World locally to make your own leads up.
You can use those 3 way junction anywhere you need to go down from two input to one output.. They are just common connectors, one goes to male the other goes to female...
With 4 panel you just need to double what's in this picture. You then add another two of those connector and you are back again down to 2 wires only ... It is a very common connectors and they come in different size and polarity to accommodate most situation of installation..
