Just in case there's any confusion with the strike/mash temperature, be sure to have your strike temperature at least about 10 degrees above your mash temperature. For your first all grain, I'd strike with your 4.5 gallons at about 15 degrees above your intended mash temperature. Stir the mash thoroughly, then measure your mash temperature in multiple locations. If you're too hot, add cold water to your mash and stir until you hit your desired mash temperature. It is much easier to be a little high on your mash temp and add cold water than it is to be too low on your mash temp and add hot water.
The sparge temperature is not as critical as the mash temp. Shoot for your 168 (I usually add my sparge water at 175 or so and let it settle in the mash [I batch sparge]), but if you are some degrees off don't worry about it.
The reason 168 is recommended is because at temperatures above 168 enzymes in the mash start to denature. In layman's terms, you "lock in" the fermentability of your wort. Try not to sparge above 180, where you risk tannin extraction from the grain, and I wouldn't worry too much about it. If you're below 168 for your sparge the only things you'll really run into are:
a) a potentially more fermentable wort than you intended
b) a slightly lower efficiency into the boil kettle (sugars from the mash are more easily dissolved in warmer water and will be pulled out of the mash during lauter more effectively.
Both of these previous issues are negligible for your first all grain.
The key thing to focus on tomorrow is...TAKE NOTES! The more information you gather the more consistent your results going forward will be. My note book has space to write down:
-Strike water volume and temperature
-Mash temperature / pH (don't worry about pH yet...also, take the temperature both at the beginning and end of the mash)
-First wort runnings volume and gravity reading
-Sparge water volume and temperature (and what temp it settles at in the mash if you desire)
-Total wort volume collected and gravity reading.
If it's feasible, pick up a few pounds of DME to have on hand. Expect that on an average 1.050 beer, your pre-boil gravity will be around ten points lower than your starting gravity. If you're a few gravity points low, add some DME and compensate. If you're a few points too high, you can water your wort down before the boil. Until you work out your system's efficiency you'll need to play around a bit with the numbers if you want to hit a specific target.
If you haven't already read Palmer's How to Brew, look it up online and read the all grain tutorial/walkthrough. It's a great resource.