At this time, and going forward, the best place to find any information regarding optics and cable support for any Juniper product would be the Pathfinder Hardware Compatiabily Tool (HCT). The list of supported items for any product change so fast that trying to keep up via on-line documentation is too difficult. Use this tool always going forward, as any other documentation is very likely outdated.
So for EX4600 the link is: https://pathfinder.juniper.net/hct/product/#prd=EX4600
You will see two 4x10GE break-out cables listed - QSFPP-4X10GE-SR (listed under 10GE) and JNP-QSFP-4x10GE-LR. Both of these cable terminate with fiber connectors, NOT copper. I believe there may not be such a 40 to 4x10GE breakout cable for copper.
I do know that ProLabs does have some 10GE Copper options available. You can look at:
http://www.prolabs.com/newproduct/10gbase-t-sfp-copper-transceiver/
http://www.prolabs.com/products/optical-transceivers/juniper/SFP_Plus/EX-SFP-10GE-T-C/
These may help you and ProLabs (not Juniper) states these will work with various vendors, including Juniper. YMMV.
You may also be able to find 40GE to [single] 10GE SFP+ adapters out there - google search for these. The disadvantge of this approach is you only get 1x10GE per 40GE interface, and then you probably need to use a 10GE SFP+ Copper Optic if you want 10GE Copper. These adapters allow 10/40 operation, just as SFP+ allow 1/10 operation. I am not aware of any devices whose native 40GE also supports 10GE or any format.
As I have stated a few times before on this forum, your best approach is to work with your Juniper [value-add] partner or Juniper account team to find all your options.
Hope this may help and good luck.