Preorders from AT&T start today for the Samsung Galaxy S 4 with a ship date of April 30. Which we assume makes the release date May 1, May Day.
The Samsung Galaxy S 4 will released from T-Mobile on May 1. Other carriers haven't announced release dates, yet. 
If you can't wait independent U.S. retailer of unlocked smartphones, Negri Electronics states that Samsung Galaxy S4 has an estimated availability of April 22. Compare HTC One vs Samsung Galaxy S4. A leaked document suggested that Verizon customers will have to wait until May 30 for the Samsung Galaxy S 4.
The Samsung Galaxy S4 features top of line specs including 5" Full HD Super AMOLED touchscreen, 2GB RAM, 12MP camera, microSD slot, Wi-Fi and Temperature/Humidity sensors.
Special software features of the Samsung Galaxy S4 are S Beam, S Share, S Play, Smart Pause, eye-scrolling, S Heath monitoring, Home Sync, Dual Video Call, Air View/Gesture, Sound/Shot, Group Play and S Translate.
The Galaxy S4 will be launched on 327 carriers in about 155 countries.
As we noted previously it may behoove you to wait for the Samsung Galaxy S 4 for save money, have more choices, more accessories and more stable software.