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Edited your original post for clarity.
I don't think this will match anything:

                <field name="severity" operator="and">
                        <value operation="eq">3</value>
                        <value operation="eq">4</value>
                </field>

It should be an operator="or" in this case, at least if I'm understanding sk122323 correctly.
Also, everything in the filterGroup must match (e.g. product = X AND severity = Y).

That basically means you'll need to create two different filterGroups (one with the blades you want to send based on priority and one with the blades you want to send irrespective of priority).
Whether you can put that in one filterConfiguration.xml or you'll need to configure a second export to the same server with the other filterConfiguration, I'm not sure.