Galaxy S4 at T-Mobile could be acquired starting April 24th, but this carrier hasn't yet announced a day from Which it Will start taking preorders, anyway, for those who want to make great deals in this period of time, you could ask to benefit from the new Choise Plan available at T-Mobile.
Unlike Sprint and T-Mobile, AT&T has already started to take preorders and this new smartphone will be available for sale from April 30th. The others big carriers like Verizon still has not officially announced any plans for the sale of S4, but we already know that Samsung has officially announced several on several occasions that by the end of the month, this new smartphone will reach all the major carriers, so very probably to Verizon, too.
My advice to you is to check all the offers before buying this high-performance smartphone, to learn how you can benefit from substantial bonuses at preorders.