FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
BGP four-byte AS Path (RFC 4893) has been supported since FortiOS 4.0MR2.

For more information about BGP 4-byte AS Path Support please refer to the Fortinet Technical Guides available in the Fortinet Document Library:
  • What's new in FortiOS 4.0MR2
  • Dynamic Routing (FortiOS 5.2)
A BGP router defined with a local 4-octect ASN and peering with another (old) router capable of only 2-octect ASN (that is: not supporting RFC-4893) will send an 'Open' message containing the 'My AS' field with the dedicated value "23456" (see RFC).

The impact this will have on the 'old' BGP speaker is that this 'old' BGP speaker believes it is peering with Autonomous System ‘23456’ (a two byte AS) instead of the real 4-octet AS.