Hi @gmb ,
here is the way how to mount Zynthian from scratch, not as kit.
You have Raspi4 a Hifiberry, so you need ZynScreen v1.5 and Controllers Set v3 & v4.
You will haven´t midi output and input by DIN connectors (becouse zynaptic module is not avalaible). For midi connection you will have only throu USB on raspi4.
On your side is make conection beetwen raspi headers and ZynScreen (courefull with correct possitions of pin). I have experience with coonnection by single wires with Dupont conectors or IDE cable, look at my experience with Zynthian from scrath. For this you can use Ribbon Bus Cable, here is litle limitation in lenght, but when you will be make good house box for all parts is possible using it.