What format do I need the certificates in to load them using Webstar V4?

The certificates must be provided in a single chain file in Netscape format
Please request this from support@comodo.com

What format do I need the certificates in to load them using Webstar V5?

The certificates must be provided in a single text file in this order:
The customer certificate
ComodoSecurityServicesCA.crt
GTECyberTrustRoot.crt
And there MUST be a blank line between
-----END CERTIFICATE-----
and
-----BEGIN CERTIFICATE-----

Error: "bad unsupported format" when importing certificate.

This error happens when using the Certificate Extractor utility. If you do not use the extractor to import the certificate, it should work OK.

Error: "Netscape cannot communicate securely with this server. No common encryption algorithms"

This error is a known bug in earlier versions of 4D Webstar Server Suite/SSL. You should upgrade to the latest version, and make sure all fixes and updates have been applied.
If you are using the latest version, check that your SSL settings are correct. The server should be listening on Port 443 for SSL connections, and have the correct certificate and key file specified. Enable all ciphers. If you have not set up your server correctly for SSL, you may receive this error.