I have an LG Viper and I get an upgrade in April 2015 what will I get next?

Will they upgrade me to a LG G3 (comes out next month) or is it optional?

I'm on sprint and I currently have an LG Viper, but I had an IPhone 4 last year so would that count towards my choices?

The will offer you a whole bunch of upgrades, and the LG G3 is the best choice right now yes.