Super Saiyan 4 vs Super Saiyan Blue is being reported to appear fighting in Dragon Ball Heroes. Dragon Ball Heroes it's an official non canon Dragon Ball videogame and manga. Which transformation is stronger?
Note: before marking the topic as "opinion based" notice most transformations in Dragon Ball are officially ranked in order of power through the anime, manga and official guides and websites.
(with the exceptions of Super Saiyan Rage and Legendary Super Saiyan / Super Saiyan Berserker that nobody can assure exactly where they are)
(Oozaru < SSJ1 < SSJ2 < SSJ3 < SSG < SSB / SSRosé < UI)