I am making a game in Diamondfire and when I reduce the players max hp attribute to 4, the low hp effect will play. (despite me being at "full " hp)

You can configure it using /cmdclientconfig. Normally it uses the maxHealth option, which is configued to 6 (which is 3 hearts). There is another option called useHealthPercentage, if you set it to true it will use the maxHealthPercentage (in this case the heartbeat effect will appear once you have 30% or less health. You can configure the values as you want to.
Hope that solves your issue.