Darklone said:-4? Heavy shield?
In 3.5 shields got renamed from large and medium to heavy and light (respectively). So the shield that gives you a +2 shield bonus to AC is a heavy shield. Because of its size, it's considered a "one-handed melee weapon." If you go with anything but a light weapon in your weapon hand, you will have two one-handed melee weapons which means you suffer the -4 penalty to both attacks. This is only for TWF.