Power levels in Dragon Ball Z have long been a point of contention among fans. There are tons of surprises in Kakarot's power level ranking, often marked differences in power of certain forms or fusions between the manga and the anime, and even the official statements on the power multipliers for various forms can be confusing and contradictory at time. Where things get real confusing is when considering Dragon Ball: GT and the fan favorite form that came with it, Super Saiyan 4. Due to its non-canon status, there's no real answer to which form is more powerful, but Dragon Ball Z: Kakarot could change that.
It should be stated that, at the moment, there's no plans, rumors, or even leaks of a Dragon Ball: GT DLC for Dragon Ball Z: Kakarot. It's still technically possible, as neither Bandai Namco nor CyberConnect2 have come out and said that it wouldn't happen, but if it does come fans will need to wait a long time. Even so, if Dragon Ball Z: Kakarot were to add the Super Saiyan 4 form, it's interesting to think how it would compare to the new form coming in DLC 2, Super Saiyan Blue. There's a lot to consider here based on how the forms are shown in their respective shows, but the answer seems clear after looking at the data.
Super Saiyan 4 in Other DBZ Games
Super Saiyan 4 has made it's fair share of appearances in other DBZ games, as regardless of how fans feel about Dragon Ball: GT, this transformation is a fan favorite for many. Even so, it hasn't cropped up in games alongside Super Saiyan Blue until recently, with entries like Dragon Ball Xenoverse and Dragon Ball FighterZ. The biggest issue is that neither of these titles really give easy numbers that denote which form is more powerful thanks to the nature of their genres.
The first issue is that in neither of these cases can Goku or Vegeta choose to go Super Saiyan Blue or Super Saiyan 4. They are separate characters altogether with different stats rather than transformations to tack onto a base form. What's more, as a fighting game, all of Dragon Ball FighterZ characters are meant to be balanced against each other for the sake of fun. There's no canonical way that Yamcha would stand a chance against Vegito Blue, but that matchup could still result in Yamcha winning depending on player skill. In the case of Xenoverse, the exact stats of non custom characters is hard to determine, so that isn't much help either.
Dragon Ball Z: Kakarot Super Saiyan Transformations
Throughout all of Dragon Ball Z: Kakarot's many Super Saiyan transformations, a pattern is held that makes things really easy to understand. The Super Saiyan 1 transformation acts as a benchmark for the rest of them and sets this trend early on. The pattern is that the highest level (level 3) of a transformation will always provide the same power boost as the lowest level of the following transformation. This means that level 3 Super Saiyan is equal to level 1 Super Saiyan 2, for example.
When applied to DBZ: Kakarot's DLC 2, Super Saiyan Blue will most likely provide a 250% boost to all stats at level 3, as the currently attainable power is Super Saiyan God which caps out at 200%. These numbers don't represent the actual multipliers at all, but they do well within gameplay to demonstrate the crazy power difference. The trouble with including Super Saiyan 4 in a future GT DLC is figuring out where in this power spectrum it fits. At the very least, it would have to provide the same power boost as Super Saiyan God given that both forms come after Super Saiyan 3, but could it surpass even Super Saiyan Blue?
Dragon Ball Z: Kakarot - Super Saiyan 4 DLC
When introducing a new form, it has to be different in some way from other forms or else there'd be no incentive to use it. For example, Kaioken and Super Saiyan both provide the same stat benefits, but one drains health whereas the other drains Ki. Even among the direct hierarchy, the forms are different. It's a minor annoyance at most, but Super Saiyan 3 drains more Ki than Super Saiyan at the expense of more power, so in a sense there's a tradeoff.
It seems likely then that Super Saiyan 4 wouldn't just be a carbon copy of Super Saiyan God, as that simply wouldn't make any sense. There'd be no incentive to use it over Super Saiyan God, and absolutely no reason to use it over Super Saiyan Blue. Along the same line of thinking, it couldn't be the exact same as Super Saiyan Blue either for the same reasons, but that begs the question of how else it could be differentiated. Just making it stronger than Super Saiyan Blue wouldn't work as there's a lot to testify that the godly Ki of SSB is at least as strong (if not stronger) than SS4.
Super Saiyan Blue VS. Super Saiyan 4
Without getting into the knitty gritty details of the two forms' clash for power, it would be a safe bet to say that Dragon Ball Z: Kakarot wouldn't place Super Saiyan 4 above Super Saiyan Blue. If anything, the two forms would be extremely similar in terms of power boost and Ki drain but have different combos or provide access to different techniques. This would be an interesting way to avoid trivializing either form, allowing players to pick based on looks and abilities instead.
Another possibility, though this seems improbable, is that the two forms will boost stats differently. At the moment, every transformation in the game provides a flat percentage boost to all stats, but it's possible that Super Saiyan Blue could boost Ki Attack more while Super Saiyan 4 boosts Melee Attack. This would even further differentiate the two forms and work to solve the debate among fans about which is stronger. It could be that they are both similar in strength but with different focuses. Again, this is all speculation and a Dragon Ball Z: Kakarot GT DLC is a long ways away if it is ever going to happen, but it would be interesting to see how CyberConnect2 handles this dilemma.
Dragon Ball Z: Kakarot is available now on PC, PS4, and Xbox One.