I don't recall that can be done in FCE, at least not directly.
What you could do, however, that would achieve the same result, is this:
Let's call your existing sequence (the one that contains your original 2 video + 4 audio tracks) "Sequence 1"
Create a new sequence; let's call it Sequence 2.
Then, in the FCE Browser, select Sequence 1 and insert it into Sequence 2's timeline just the same as you might do with any ordinary video clip. This is called nesting the sequences.
Now, in Sequence 2 you can treat Sequence 1 just as if it were an entire clip by itself and you can insert a new clip at the beginning of Sequence 2 and as a consequence Sequence 1 will be shifted in its entirety without affecting the individual tracks that are in Sequence 1.