Hello,
you need to configure an EtherChannel between your switches, here are the configs (this is for ISL trunks):
4507
interface FastEthernet3/2
switchport trunk encapsulation isl
channel-group 1 mode desirable
!
interface FastEthernet3/3
switchport trunk encapsulation isl
channel-group 1 mode desirable
!
interface Port-channel1
switchport trunk encapsulation isl
3750
interface FastEthernet0/26
switchport trunk encapsulation isl
channel-group 1 mode desirable
!
interface FastEthernet0/28
switchport trunk encapsulation isl
channel-group 1 mode desirable
!
interface Port-channel1
switchport trunk encapsulation isl
HTH,
GP