I don't think it supports anything but dot1q so it's on by default.
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6021/ps9310/Data_Sheet_Cat_4900M.html
interface GigabitEthernet2/9
switchport mode trunk
!
interface GigabitEthernet2/10
!
interface GigabitEthernet2/11
switchport access vlan 172
switchport mode access
!
4900M#show ver
Cisco IOS Software, Catalyst 4500 L3 Switch Software (cat4500e-IPBASEK9-M), Version 12.2(40)XO, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2008 by Cisco Systems, Inc.
Compiled Tue 22-Jan-08 21:39 by prod_rel_team
Image text-base: 0x10000000, data-base: 0x11D8D84C
ROM: 12.2(44r)SG
Darkside Revision 0, Jawa Revision 11, Tatooine Revision 140, Forerunner Revision 1.71
4900M uptime is 7 weeks, 19 hours, 4 minutes
System returned to ROM by power-on
System image file is "bootflash:cat4500e-ipbasek9-mz.122-40.XO.bin"
This product contains cryptographic features and is subject to United
States and local country laws governing import, export, transfer and
use. Delivery of Cisco cryptographic products does not imply
third-party authority to import, export, distribute or use encryption.
Importers, exporters, distributors and users are responsible for
compliance with U.S. and local country laws. By using this product you
agree to comply with applicable laws and regulations. If you are unable
4900M#show inter trunk
Port Mode Encapsulation Status Native vlan
Gi2/9 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi2/9 1-4094
Port Vlans allowed and active in management domain
Gi2/9 1,172
Port Vlans in spanning tree forwarding state and not pruned
Gi2/9 1,172
4900M#
