No-one here can tell you exactly what will happen, because we can't see your hair.
I'm guessing you've had bleached highlights and the other dyes have then stained your hair, which is why it's lighter. The B4 should remove any build-up, but if your hair is stained or if it's bleached it won't really do too much, if it does anything. (I can't see how your hair would be stained by semi-permanent stuff, but IDK what you've used.)
Go to a hairdresser and explain your situation, they'll know what to do. The B4 might just make it worse, don't risk it.