Unlock in what way? If it's a Verizon S4, it'll work as is if you get the Verizon Straight Talk kit (the one with the red map). Go to Straight Talk Compatibility Test to see if Verizon will accept your handset using a ST account. (There are some restrictions, like if it's been on a Verizon prepaid account.)
Verizon S4's are SIM unlocked. You can use it on Straight Talk using service from Verizon, AT&T, or T-Mobile. The Straight Talk BYOP kit has SIM cards for service from all 3 carriers. They'll all work on a Verizon S4. No unlocking is necessary, as it never was locked.