Hello,
Maximum number of VPC members in nexus is 16 per VPC.
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/configuration_limits/limits_503/nexus_5000_config_limits_503_n1_1b.html.
ASA with the software 9.2 and higher supports16 active ether channel members. Just remember on ASA, you only need to configure regular port channel because VPC is transparent to upstream and downstream devices.
"You can now configure up to 16 active links in an EtherChannel. Previously, you could have 8 active links and 8 standby links. Be sure that your switch can support 16 active links (for example the Cisco Nexus 7000 with F2-Series 10 Gigabit Ethernet Module)."
http://www.cisco.com/c/en/us/td/docs/security/asa/asa94/configuration/general/asa-general-cli/interface-echannel.html
So there is no issue with configuration 4 port channel members.
Hope it helps,
Masoud