If I take my sims card out of my iPhone 4 and put it back, will I lose anything?

Will I lose all my data or anything if it's the same sims card going back to the same iPhone 4

No. The data is saved on the device itself.

Just make sure you turn your phone off first and you should be fine

No. The SIM card is just info about the phone lien your phone number. It directs and receives calls. It connects you to the network. You don't need a SIM card for iPhone to be functional. It's probably a good idea to have one though

Iphone 4 does not save any information on the sim card unless you download an app from Cydia "SIMANAGER" or something like that. All data is saved on the phone.