Re: QFX5100 - EX4300 VCF topology - spine/leaf or mesh
Virtual Chassis (VC) and Virtual Chassis Fabrice (VCF) are 2 completely different architectures, with both built off of same underlying protocols/etc.
VC can be a full mesh of up to 5 switches, just using the 4 x 40GE interfaces on EX4300 and 4 of the 6 x 40GE (QSFP+) interfaces on the QFX5100. You just need to config these interfaces to operate a VCP (Virtual Chassis Ports) vs standard Ethernet ports. These interfaces can be one or the other. By default 1st 2 ports on EX4300 default to VCP, while other 2 default to Ethernet. You can check status of which interfaces are configured as VCP via - show interface vcp
I believe this is probably what you want to do - make the 4 switches into a full mesh VC. You'll need 3 x VCP on each switch to do this. You'll find plenty of documentation on how to set up a VC; I would suggest you use pre-provision mode, this is generally the best option, especially in your case as for this VC the QFX5100 MUST be both the Master and Back-up RE.
VCF is more spine and leaf like, with the spines used to just forward traffic between leaves. In general, the Spine (2 or more) need to QFX5100-32Q, vs QFX5100-48S.
Both VC and VCF allow for a single point of management for all of the switches. VC scales to a maximim of 10 switches, while VCF can scale to 20 (and under some circumstance, even larger).
You should be able to find LOTS of documentation regarding VC and VCF, but as I stated earlier, I am 99% sure for you, the best choice would be VC.