There is a pranayama technique called Samma Vrtti which translates to mean Equal Breathing. It is a way to pull your attention to your breath, which can provide a focal point for your mind during your meditation or relaxation.
I suggest starting with noticing the current length of your inhale and exhale. This will allow you to maintain a comfortable breathing pattern with softness and ease. Breathe in and out via the nose. If you counted 4 seconds for the inhale and 5 seconds for the exhale, try using the smaller number for your equal breathing exercise. A larger number is NOT better than a smaller one. Think of it as achieving balance, more than a very deep breath.
Begin with your inhale and mentally count to yourself the duration you selected. Use the same on the exhale. As long as you are finding this is matching your natural rhythm, then continue with this number for a few more breath cycles.
Eventually, leaving the mental count so you can step away from the “thinking” mind and just feel the duration of the breath cycle. Continuing to remain relaxed with a soft, subtle and smooth breath.