The most common answer you will get to this question will be to ditch the 1x15 for another 4x10 as mixing drivers is not considered best practice here on Talkbass.
However, if your head is only capable of running at 8 or 4 ohms, you won't be able to run two 4 ohm cabs with it as that would result in a 2 ohm load.
If you want more volume, a bigger cab (i.e. more speakers) would be the best route to take, such as a 4 ohm 6x10 or 8x10.
Hope that helps