Hi, 
I would like to know how to change the state patrol backup to spawn Police 4 instead of policeb. Is there a way to do that?

You need to edit the agency.xml file located in lspdfr/data. Within the file you'll find a section like this

Spoiler

<Agency>
        <Name>San Andreas Highway Patrol</Name>
        <ShortName>SAHP</ShortName>
        <ScriptName>sahp</ScriptName>
        <Inventory>default</Inventory>
        <TextureDictionary>s_m_y_hwaycop_01</TextureDictionary>
        <TextureName>decl_diff_001_a_uni</TextureName>
        <Loadout>
            <Name>Motor Unit</Name>
            <Vehicles>
                <Vehicle>policeb</Vehicle>
            </Vehicles>
            <Peds>
                <Ped comp_uppr="1" comp_decl="1" comp_lowr="0" prop_head="0" helmet="true" inventory="biker">s_m_y_hwaycop_01</Ped>
            </Peds>
            <NumPeds min="1" max="1" />
            <Flags>
                <Flag>RespondsAsBackup</Flag>    <!-- Makes the unit eligible to be called as either Code 2, Code 3 or Pursuit backup. -->
                <Flag>SpawnsAmbiently</Flag>     <!-- Makes the unit eligible to spawn ambiently - e.g. on patrol in the area or outside of police stations -->
            </Flags>
        </Loadout>
    </Agency>

 

Alter <Vehicle>policeb</Vehicle> so it's <Vehicle>police4</Vehicle> and you're good to go.