-- SSL handshake error:
-- Error: com.cyclonecommerce.tradingengine.transport.UnableToConnectException:   javax.net.ssl.SSLHandshakeException: java.security.cert.CertificateException: Unable to validate:
-- Interchange and Partner not presenting same certificate chain, but both a 3 cert chain and 4 cert chain show as valid
-- logs show: ERROR (ManagedX509TrustManager.validate:242) - Unable to validate:

There are many causes for an SSLHandshakeException, Unable to validate.  This KB only talks about one of them:  Logging shows a difference in the trust chians; or looking at the certificate in the UI on your sytem shows a different trust chain than your partner sees on their system.

Your partner may have provided a .P7B with the complete trust chian as shown in their system, but when you import it into Interchange, you see a different trust chain that ends in the partner provided leaf cert.

Interchange will build a trust chain from the leaf cert up to a CA root by finding the first certificate that matches for each level that is found in the database.  If more than one certificate is loaded that satisfies the conditions, it will use the first one it finds.  If the cert Interchange is using is deleted, Interchange would then start finding the next cert that matches the chain. You can use the Manage Certs page to find and delete the unwanted certificate.

** NOTE, be sure that your other partners are not using certificates that reley on the existing certificate.  If you delete it for your new partner's sake, your could break existing partners.
Use caution, back up any cert before deleting it.   Talk with your partner and with Axway support if in doubt.