Explode to max dice once?

Is it possible to get  a macro to roll a dice, and if it's a certain value you add a number? An example is rolling a d4 and doubling the dice if it's a 4, so on a 1-3 it adds the modifier as usual, and if it's a 4 it goes to 4+4+mod

Unfortunately no. Roll20 dice cant do conditional effects, like "if roll = x, do something"

You can do something like this with a rollable table at the free level, but you'd need a new table for each such special case. Your table would be made up of entries like: 1 2 3 8 ...and then you could add the modifier to that. This table would specifically be for "d4 doubling on 4". You'd need a different table for "d6 doubling on 6", or "d4 with +1 for 3 and +2 for 4" ... etc... etc...