@miracullix So just bridge the ports together and give it a try. You can always "undo" what you setup - go in reverse order to tear it all apart.
Under Interfaces, select the Bridge button. In there, click the Add button. In there add the 2 ports you want together (use the shift key on the keyboard to select multiple ports) and then click the Save button. Keep in mind, the only interfaces you can add to a bridge are "enabled" interfaces. In other words, they have to be active. I think the 4100 comes with all interface ports enabled.
So, now that you have a bridge added, you have to enable it and set it up. Be careful here, I think you could inadvertently lose your LAN connection and the IP address range you already had on it.
Long story short, I don't believe you can simply click a couple of buttons and add another available interface to a bridge. There's a little bit of setup, and some pretty good setting tweaks. And, obviously the performance hit. So, that's why it's said to just add a switch to keep it simple.
Hope that helps...
