Hi Dan and welcome to TFF!
What MW says is true. It can come as quite a shock if you've read some things about cycling and perhaps taken some advice from an LFS and feel like you are being careful with your new setup, but then find here that there is this whole other world of understanding about fishless cycling and what the truly important parameters are at the core of the hobby.
Since you've found yourself in a "Fish-In" cycling situation without really knowing what the alternative would have been, it makes sense to think about what the options are at this point. The option that would potentially save you the most work in the long run is also the one that can be so awkward that most people don't do it. That its, work on your store manager to take the fish back so that you can prepare your tank properly and be ready for them. It can be a painful process and take a lot of talking but if you can figure out some way to re-home these 4 initial fish, you could get them back later and make life easier on yourself in the meantime.
The other option, as MW has said, is to proceed with the Fish-In cycle. The immediate need in either case will be a liquid-reagent based test kit. Many here use the API Freshwater Master Test Kit and there is also a similar one called the Nutrafin Mini-Master Test Kit. These kits have small bottles of reagents used to test your tank and tap water for ammonia, nitrite (NO2), pH and nitrate (NO3). Test kits like these should probably be the number one thing beginners are concerned about after they get a tank, filter and heater.
The links in MW's post should have discussions of the nitrogen cycle, setting up a tank and the add and wait process of fishless cycling. The more you read and reread of all these, the more it will all start to make sense, or at least lead you to ask questions that will help you more.
Following the Fish-In Cycling route, the problem will be trying to keep ammonia and nitrite below 0.25ppm and hopefully at zero. Often this takes large (50% or more) water changes daily, sometimes two a day or sometimes later only one every two days, but for the 4 week or so period while you are growing the two species of bacteria in your filter, it can add up to a lot of water changes. For a 29 gallon, it will almost be imperative to have a Python type hose system to be able to pull this off.
Good luck and I know the members here will be glad to help you if you ask,
~~waterdrop~~