is it possible to combine the 4th gauge spell and the 3rd gauge spellIt is completely possible; take note that you don't need to use up a gauge to cast a spell. All you need is to get your B-Point up to 100000/300000/500000. Once you cross those marks, a level 4 spell will automatically occur, and THEN you can cast your level 3 spell. If you cross 500000 but you're in the middle of a level 4 spell, a level 3 spell will automatically occur, and then you can release ANOTHER level 3 spell.
Put your B-Points higher, and your gauge fills up swifter and swifter. In higher difficulties (maybe even Easy, given that both players are doing really good), Marisa has the ability to infinitely cast level 2 spellswhen she hits 999990, provided that the player can get 100+ combo for every level 2 spell she releases.