The Athlon X4 880K has an unlocked clock multiplier, which means it is possible to overclock it just by adjusting its clock multiplier.
In quick tests, we manage to run the Athlon X4 880K with stability (we tested stability with Prime95) up to 4.4 GHz, with 100 MHz base clock and x44 multiplier, with default voltages. At 4.5 GHz, we don’t get full stability.
Just keep in mind that we reached this overclock without changing any voltage configuration. So, if you have patience to adjust them, and other advanced options, you may reach higher clocks.