Lower the spanning-tree priority on both 7K to lower than 4510.spanning-tree vlan 1-4094 priority 4096 !(Must be lower than 4510)or spanning-tree vlan 1-4096 root primary
Depending in the platform and IOS version, you can change in configuration mode by:interface Port-channel X switchport mode trunkin older version IOS, you may also have to set the encapsulation: switchport trunk encapsulation dot1q
