I wouldnt keep more than 3 guppies in a 4 gallon tank.
You have two options, either buy a bigger tank that will hold all these fish, preferably two tanks so you can seperate the sexes so they stop breeding and producing MORE fish! You will need at least 30 gallons.
OR you can take most of the fish back to the shop or find another home for them, where they wont be overcrowded. And keep just a few male/female guppies.

If you dont do any of that you will end up with a tank of dead fish, it doesnt matter how many water changes you do or what size filter you have. That many fish (and all the babies that are coming) is just far too much for your tank/filter to cope with.

In the future, dont buy fish because its 'cheap'.. I hope you've learnt from this.