The Cisco 3524XL doesn’t support PaGP or LaCP, you simply configure etherchannel by adding ‘port group N’ to each interface. The port group takes the configuration of the first interface in the port group

! Cisco 3524XL
interface FastEthernet0/1
 description uplink to 6509
 port group 1
 switchport trunk encapsulation dot1q
 switchport mode trunk
end

The 6509 supports more dynamic protocols, and will try to use them unless you specify ‘switchport nonegotiate’ on the portchannel interface, which is key. Otherwise everytime you turn on ‘channel-group 4 mode on’ the ports will go down on the 3524XL and the ports on the 6509 will go into the ‘err-disabled’ state until you ‘shut’ / ‘no shut’ them.

! Cisco 6509
interface GigabitEthernet7/7
 description sw03 - rack 3
 no ip address
 switchport
 switchport mode trunk
 switchport nonegotiate
 channel-group 4 mode on
end

interface Port-channel4
 description sw03 - rack 3
 no ip address
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport nonegotiate
end