I'd go for about 4 minutes - if you talk fast you'll fall within the boundary, if you talk a bit too slow (unlikely) you'll still fall within that boundary. It might be worthwhile asking your teacher in advance what they're looking for, but chances are you'll break down your speech as follows:
0-20s introduction
20-60s item 1
60-100s item 2
100-140s item 3
140-180s item 4
180-220s perhaps some discussion of your texts
220-240s conclusion
OR something like that - I rarely break down my speeches in that much detail so I just guessed the times. I tend to contrast / discuss my texts as I go (which is a much better way of doing things - you can talk about ideas, context and all that) but if you're struggling with synthesising then I would talk about them one by one and spend a good chunk of time discussing them.
When I say 'talk about one by one' I don't mean 'describe in minute detail', by the way - you want to briefly cover: the themes, what kind of medium (and what that means??), the author, maybe some background of the construction of the text
And then when you're comparing and constrating you can talk in detail about techniques and how they create similar or constrasting ideas.
General speech tips.. well I'm not a speechwriter but I'll try and sum it up in another post (hopefully today .. but I'm tired)