Sorry, but the above post is wrong. ELRS and CC2500 are not the same.
With a cc2500, you'd be able to connect to a limited number of rc receivers - specifically ones made by (Frsky, S-FHSS, Radiolink, Graupner HoTT, Corona, and Hitec). (Possibly some others, but not a super extensive list)
With the 4-in-1, one of the 4 is a cc2500. The others are NRF24L01, A7105 and CYRF6936. The 4-in-1 will allow you to make your transmitter work with A LOT more rc receivers, including most of the ones that Bind and Fly (which are often DSMX) and RC Toys use. With a 4-in-1, you have one transmitter to rule them all. In my opinion it's very worth the extra ~20-30 bucks to get a transmitter with a 4-in-1 module. There's a pretty good list of the protocols a 4-in-1 transmitter can talk to here -
DIY-Multiprotocol-TX-Module/Protocols_Details.md at master · pascallanger/DIY-Multiprotocol-TX-Module · GitHub Edit - since you specifically asked about flysky, that protocol uses the A7105 chip, so the 4-in-1 would work for you.
ELRS is one of the "New Generation" protocols that have lower latency and transmit 5-10x further. It's pretty amazing, but thus far not widely adopted yet. These transmitters/receivers have extra modern functionality like bluetooth and sometimes wifi, and you'd likley use your phone for configuration. I don't have any ELRS stuff yet, but from what I've seen, you set up a binding passphrase instead of holding buttons, etc like the traditional receivers. The price on the ELRS receivers is really agreeable, but you'd lose out on having the one transmitter to rule them all, if you want to use it for bind and fly models, or use it for other toys.
My main transmitter is a 4-in-1, and I'm considering getting a ELRS module to plug into the bottom bay to play with it, and because it might be the most affordable way to expand my fleet (at last as far as the price of receivers go)