Cookie data size exceeds 4K - But its 'only' 1100 bytes encrypted
Fred is right. session.inspect is not going to show you the exact size of the cookie string length. You could use tcpdump to get a better idea of its size;
tcpdump -s 1500 -A host and port
Then make the request and check out the actual ascii dump of the cookie data.
lal00 is likely onto the real underlying issue