Hi,

you may achieve it through code customization. In Odoo the sequence of purchase order is defined according the special object 'ir.sequence' (see the function create of 'purchase.order'). You may re-define the method next_by_code:

1. add column "user_id" (res.users) to "ir.sequence"

2. define a sequence for each user. Here you may also indicate some prefix (like 'Brown/' to get  "Brown/2121" instead of "PO2121"). In case you prefer a 1000 increment, put in sequence the 'number_next_actual' a required start number (e.g. 6000). I think, prefix is better than just 1000 increment, since no further troubles appear when there are more than 1000 orders per one user.

3. in the method next_by_code beside checking a company, check for a user: current_seq = self.search([('code', '=', sequence_code), ('company_id', '=', self.env.user.company_id.id),('user_id','=',self.env.user.id)])