Hi there, and welcome
Congratulations, you have a pretty straight forward switchover ahead of you. There are two sets of wires, necessary and optional.
Necessary wires are all connected to the 4 zone controller, there are:
1) Two White wires, common. Connect it to any of the C terminals of your new Gen 2 rachio. I recommend you connect one wire per terminal.
2) Dark red thick wire, Master valve. Connect it to the M terminal of your new Gen 2 and enable Master valve within rachio software.
3) Zone wires, Yellow through thin red wire. Connect them to zones 1 through 4 of your new Rachio.
Now for optional wires:
1) Thick orange wires are power input, they should not be connected to your new controller. Use the new power adapter which came with your Gen 2 instead.
2) Thin red and black cables connected to 24VAC terminals. These are power wires for a wireless rain sensor. They can be connected to SC and SP terminals of your new Rachio.
3) White and Green wires connected to SENS terminals, these are the signal wires from your wireless rain sensor. You can connect white wire to SC terminal (you will end up with two wires connected to this terminal, that is ok; I recommend you twist the wires together before inserting them into the terminal). Connect the Green wire to S1 terminal and enable rain sensor for S1 input within Rachio software.
That is about it, rain sensor is not necessary for Rachio since it can use wifi + internet data instead, but since you already have it, may as well use it.