On a GroupWise 8.0.1 system, you cannot set Mailbox Size Limits on the Client Options, Disk Space Mgmt tab any higher than 4 Gb, if your GroupWise user's client version is 7.x or below.
The 7.x client still only recognizes up to only 4 Gb. So, your over-the-top users (who have mailbox sizes higher than 4 Gb) have to have a custom levels set to a Mailbox size limit of 0 (zero), and a suggested Threshold setting of 100.
When you eventually get to upgrading the GroupWise clients to 8.0.1, then you can set Mailbox limits to sizes above the 4 Gb limitation of the client. The new limitation is 8 Tb.