It is perfectly safe to use all four B Strong bands simultaneously. It should be noted that using 4 bands at once can make exercises more difficult.
When people use all 4 bands simultaneously, they generally finish the session sooner (~15 minutes). We recommend that you start your BFR experience with just arms or just legs at once. Get used to the feelings and the exercises and ease your way into the training. As you become more experienced after a couple sessions, you may apply all four belts at once and incorporate more complex movements or callisthenic exercises.
It is important to be well hydrated during all B Strong sessions, and especially when you have four belts at once because you are pooling a larger blood volume in your limbs. If you ever feel light headed during any workout, stop, remove the belts, hydrate and eat something and try again later or the next day.