A 10-gallon tank would be a perfect birthing/fry tank!!!
If I had a spare 10-gallon for a birthing tank, here's what I'd do:
Clean out the tank really well with very hot water, and some oxy clean if you have it, or maybe some vinegar... spiders/lizards may have left behind germs etc...
Plants - Java moss and maybe some duckweed (I think that's what it's called... it floats) - maybe tie a bunch of the moss to a small clay flower pot.
Substrate - Line the bottom of the tank with dark colored marbles or larger, pebble-sized gravel - more hiding spots for the fry, and will make it much easier to vaccum clean
Heater - 50 watts would suffice - tank temp should be ok around 77ish
Filter - Top Fin Power Filter 20 about $15 (you could get the Powerfilter 10 for about $12, but for an extra $3 you'd have better filtration w/ the 20) OR Penguin bio wheel Power Filter 125gph for about $19. Both sufficient for your 10 gallon, I'd lean towards the penguin, only because it has the bio wheel too. Set the flow rate to medium, get a pair of nylon knee hi stockings, and put one on the filter intake tube. That will prevent the fry from getting sucked up!
Other - Hmmm, maybe someone can correct me if I'm wrong, but I think you could get 3 cories for a bottom clean up crew - I'm pretty sure they wouldn't eat the fry... but not 100% sure. Once the guppies give birth, as long as you have plenty of hiding spaces for the fry, leave the mommies in the birthing tank for a couple of days to recouperate. Oh, I'd add some salt to the tank too (table salt is ok), 1 tablespoon (melt it in hot water first, then pour into the filter or directly into the tank if there's nobody in there.
Try to set the tank up asap, especially if the look like they're gonna pop soon (though if worse comes to worse and you see one giving birth, you can always scoop her up into the birthing net thing, let her give birth, take her out as soon as she's done, leave the fry in there while you set up the tank, and then use a turkey baster to suck them up and transfer to the 10... I had to do that once) and though it won't be cycled, you should be ok with those 4 female guppies in there... maybe you can take some filter media or even some gravel from your big tank and put it in the 10 gallon to help the cycle along. Do frequent water changes, and make sure you replace any salt you take out with the water change.
Have fun, good luck!
~Tara
edited to make sense...