4 ohm speaker in place of an 8 ohm

[ppppenguin]

There is no doubt your 4 ohm speaker will work. But it will be a lot louder than the 8 ohm one next to it. And as @skyclad says, it will overload the amp. Whether the amp will mind is another question. A tough amp may well stand up to this kind of abuse, especially if you don't wind it up to 11.