JCraft X Heavy Core 4-String BEAD Tuning Electric Bass Guitar String 65-125JCraft X Heavy Core Bass strings are built in custom gauges perfect for drop and heavy tuning. They are forged with the timeless combination of hexagonal core wrapped in nickel alloy for the best tone and durability. The heavy drop 4 is for B tuning your 4 string bass. If you play core notes in a metal band, you don't have to buy a 5 string, just hook these strings into your 4 string and play along with a low B! 1st String: 0. 065 (D) 2nd String: 0.