mailbox size limit ?

[RJ45]

Hello,
on my system I have
cyrus-imapd-2.2.12

I noticed that mailboxes with quota over 4Gb does not function very well.
That is from imap clients users are advise of overquota when the mailbox 
is over 4GB of size.
let say mailbox is 10GB of quota and usage is let say 5 GB
Users are notifyed of overquota anyway like if more than 4GB cannot be 
handled. is this normal ?
any hints ?

thanks

Rick