I only use the 4-count breath in specific instances - and otherwise make a point of it to be aware of myself breathing.
When I first started out practicing 4-count breath / being aware of me breathing - I was trying really hard not to think, for nothing to come up - and to then try and do this for 21 days.
This did not work out at all lol -- because I was in essence denying who I was and where I am at in my process; kind of like being a robot but trying really hard not to be one - if that makes sense.
So instead of wanting to achieve this breathing for 21 days / x-amount of time - I realised that this point was an excercise as much as it was the entire process I am walking in itself. Because - as long as there is any form of programming inside of self: thoughts will come up, reactions will occur, breaths will be missed. And so I realised that it's to take those thoughts, those reactions - see what patterns/constructs they represent, investigate them, release them, correct them - and continue breathing - until the next breath you missed by reacting. And so, within walking moment by moment, breath by breath and taking on the skipped breaths, the moments where you react = you systematically take on your Self Programming, peel away a layer at a time until one day you don't miss any of your breaths and then you will have your 21 days of breathing and beyond.
So, cool if the metronome is assisting you with your breathing application -- but remember that it is but a bridge to support you and that you will have to stand as that support for yourself in time to come, to remind yourself, to push yourself to be here in everybreath - and that the beat/consistency of your breathing pattern is not as important as your simple awareness of you breathing.
And to also not place to much importance/emphasis on the days/time you are 'accomplishing' but to focus on the process/journey and to work with what comes up.
Enjoy the process!