It's never considered in the books that Mel can shoot fire from her fingertips, she used some sort of power to make Orell's eagle burst into flames North of the wall when Stannis invaded, but it was never explained how she done it. In the show we don't see that happen but other than that I think we have seen the majority of Mel's magic capabilities portrayed in the show, shadow babies, visions in the fire, not being poisoned by the wine etc.
So as the show writing her to be less powerful/magical than she is in the books, I don't believe so. They have definitely proved in the show what she is capable of and that her power is legit. If they were to give her too much magical powers I think they could run the risk as making viewers portray her as a sort of witch rather than a priestess who has been gifted with powers by the Lord of Light.
As for Season 4 the reason why she didn't appear in many episodes is because she doesn't appear much in the book which S4 is based on, ASOS pt.2.
The scene with Selyse had a bigger purpose though than to just state that Mel's potions are lies and tricks to draw unbelievers in until they see the real truth that The Lord of Light exists. It was to introduce the fact that Mel somehow needs Shireen once they get to the Wall and in turn she will need Selyse's help. It was sort of to set up a future plot as Mel's storyline will get considerably more important in Season 5.