Welcome to Tropical Fish Keeping forum.
The 4-foot 55g tank has lots of options. As Varkolak said, first thing is to know your water parameters out of the tap. This may determine what you need in the way of aquascaping, filters, and light. Different fish have different preferences respecting water flow (the filter), plants have light needs which also affects fish, and the type of substrate (gravel, sand) may depend upon the fish. And selecting fish based on your water parameters is safer and easier than attempting to adjust the water for sensitive fish.
You can ascertain the hardness (GH or general hardness, and KH or carbonate hardness, also termed Alkalinity) from the water supply folks; they may have a website. Should also be able to give you the pH too, though it is worth having a pH test kit such as the API liquid test as this is a useful test to make periodically.
I'll also mention our fish and plant profiles, under the second tab from the left i the blue bar across the top of the page. Most readily-available freshwater fish are included, and each profile gives info on minimum number needed, minimum tank size, compatibility issues, water preferences, etc.