That sounds roughly reasonable to me (the devil could be in the details of the connection of this quantity to your actual research question, but that’s your call as the domain expert).
The ROPE should certainly be informed by the domain and there is IMHO no general reasons why the same ROPE should be used for multiple different questions. You might also want to do something like actually fitting (for each sample separately) a line between the four values and take the residual sum of squares, giving you a posterior distribution for this residual, which might be easier to interpret.
Also there is IMHO nothing wrong with just reporting an appropriate posterior interval. For a positive quantity, it might make sense to not report the central interval but rather a 95% interval from zero up, i.e. P(RSS < a) = 95%
Alternatively you can build a model that forces a linear trend (by coding the levels as real numbers and treating it as continous predictor) and then do model comparison with loo between this and the more expressive model where each category can differ. (this would answer a different question, i.e. whether not treating this as a linear trend improves prediction).