Design notes:
Looking at the diagram I have a few comments on the interconnects.
Remember that when you cable up the 8 ex4550 as a virtual chassis these connections allow traffic the flow between all these switches without any other connection being needed at all. The group of 8 in a virutal chassis can be treated as if it was one physical chassis box with multiple blades.
Thus you only need to have a physical lines from an ex4550 in rack one and an ex4550 in rack 4 back to your core switch. These two physical lines would become members of an ae bundle on both the VC ex4550 and the core 8208 switch.
If two ports was not sufficent for the bandwidth you then can start adding additional ae member interfaces on both sides until the bandwidth needs are met. But the VC cables take care of getting all the traffic from the other member switches over to the uplink interfaces.
Configuration Outline ex4550 VC
For the ex4550 you will first plan the cluster and get the VC cables physically connected as you indicate on the diagram.
All devices will upgrade to the same version of Junos you want to run. JTAC has version recommendations here.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB21476
Once the VC is up and running as expected, you can setup the desired VLAN configuration for the servers. Remember that all 8 devices are covered now by a single login and just differ only by their interface fpc number. So all the configuration for all 8 switches is done is a single device login. This document reviews how the VLAN setup works.
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/getting-started-els.html
Once that is all setup you will need to create the ae trunk port on both the ex4550 VC and the core ex8208 to bring the vlans back to the core. This is also covered in the ELS guide above.