The claim form (both EDI and CMS-1500) allow up to 12 diagnosis codes to report and typically most payers allow up to 12 diagnosis codes.
It appears that this payer has a smaller limit to the number of diagnosis codes that you may report on the claim form. Most likely, they are only allowing up to four diagnosis codes, but you may want to verify that with this specific payer.
How to Fix
The diagnosis codes are entered / edited from the add / edit client form, in the section labeled Cases. Open up the edit client form for this page. At the bottom right, click the edit icon associated with the diagnosis code set. Remove however many diagnosis codes that need to be removed to meet this payers limit. Remember to save the client when done.