Require a certain level before registering for another level with Paid Memberships Pro.

my_pmpro_registration_checks.php

<?php
/*
	Require a certain level before registering for another level.
	Add this code to your active theme's functions.php or
	a custom plugin.
*/
function my_pmpro_registration_checks($okay){
	//only check if things are okay so far		
	if($okay)
	{
		global $pmpro_level;
				
		if( 
			( $pmpro_level->id == 2 && !pmpro_hasMembershipLevel( 1 ) ) || 
			( $pmpro_level->id == 4 && !pmpro_hasMembershipLevel( 3 ) ) 
		){
			pmpro_setMessage("You must be a free member first before registering for this level.", "pmpro_error");
						
			$okay = false;
		}
	}

	return $okay;
}
add_action("pmpro_registration_checks", "my_pmpro_registration_checks");