One approach you could take is to add a yes/no field to your client table that records whether that client has been exported.
Then create a select query (let's call it qryExportList) that goes something like
Code:
Select Top 400 ClientID, ClientFName, ClientLName, (add in all the other fields you want here, seperated by commas)
From tblClients
Where [Client_Exported]=0
Order By [ClientID];
Then, use the docmd.transferspreadsheet vba command to export the results of that query to a spreadsheet. You may wish to prompt the user to provide a filename to be used for the export. Checkout access help on how to use this vba function, or do a google search.
Then immediately followup that export event by running an update query that goes something this:
Code:
Update [tblClients]
INNER JOIN [qryExportList] ON [tblClients].[ClientID] = [qryExportList].[ClientID]
SET [tblClients].[Client_Exported] = -1;
Then, after you have run the update query, repeat the process and you should get the next 400 clients.