You will have to choose one feature or the other as they both consume your only bonus action. You will have either 3 attacks with the Martial Arts feature or 4 with Flurry of Blows.

PHB P78, emphasis mine:
> **Flurry of Blows.** Immediately after you take the Attack action on your
> turn, you can spend 1 ki point to make two unarmed strikes as a
> *bonus action.*

Then under Martial Arts:

> When you use the Attack action with an unarmed strike or a monk weapon
> on your turn, you can make one unarmed strike as a bonus action. For
> example, if you take the Attack action and attack with a quarterstaff,
> you **can also make an unarmed strike as a bonus action**, assuming you
> haven't already taken a bonus action this turn. (*ibid.*, emphasis mine)

Since you only get one Bonus Action in your turn you choose one or the other as indicated here PHB p 189 under Bonus Actions:

> You can take a bonus action only when a special ability, spell, or
> other feature of the game states that you can do something as a bonus
> action. You otherwise don’t have a bonus action to take. You can take
> only one bonus action on your turn, so you must choose which bonus
> action to use when you have more than one available.