You're not going to get a more detailed answer unless you state what software you use to edit -- not all video editors are created equal, some allow you to mix aspect ratios, framerates, resolutions etc. on the same timeline, and some don't.
4:3 is obviously not the same as 16:9, so if you want to use the former with the latter, you must decide whether you want black pillars on the side, or if you want to zoom in on the 4:3 content so it matches the width of the 16:9 video (which cuts off a portion of the 4:3 video at the top and bottom).
Your best bet is to take this to the forum specific to the video editing software you use, someone there who is familiar with the program should be able to give you step-by-step instructions.
One way you can do that in Edius is to create a new sequence, drop the 4:3 clip into it, bring up the Layout tool (F7), and zoom in on the video. There should be a "Fit Width" preset there that will zoom in just enough to fill the entire width of the 16:9 sequence, cropping the top and bottom as needed.