I don't disagree with the reviewers above that there are very advantageous aspects of this card over the other Big Damage Mystic spells out right now, but my enthusiasm for the level 4 Brand is marred by the same irritation I have with many of the top-level Mystic spells. Namely, the ratio of benefits to consequences gets worse instead of better between level 1 and level 4 of this spell.
In this case:
Level 1 Brand: you get +1 skill value and risk losing 1 action.
Level 4 Brand: you get +2 skill value and risk losing 2 actions.
It superficially seems like you are getting twice the boost for twice the risk. The problem is that skill boost doesn't happen in a vacuum. It's a percentage of your baseline skill, so going from +1 to +2 doesn't double your chance to hit. And since the bigger modifiers are more rare in the Chaos bag, you have even more diminishing returns in terms of odds for each skill point you add. For example, if you have +1 over the skill test, your chance of success on normal is usually around 30%. If you boost by 1, to be +2 over the skill test, your chance of success is usually around 60%, a big increase. If you boost again, to be +3 over the skill test, your chance of success is typically around 75%--a much smaller increase as that first extra point was (even though you definitely want the increase!). The exact percentage would depend on the test, your stats, and your Chaos bag, but compared to level 1, you are essentially risking 200% as many actions while getting somewhere in the range of a 20-50% increase in your chance of success.
In many of the top-level spells, you accept that you are making a bigger bet in order to get a bigger damage payout all at once. Sometimes it's worth doing 3 damage at once so that monster can't hit you back. In this case, though, you can do equivalent amounts of damage with both cards, because the amount of damage is determined by the charges you spend, not the level. You do get 3 more charges to spend at level 4, but I'm not convinced that's worth 3 more XP versus running another recharge card in your deck.