Some ferocious abilities that appear on instants and sorceries use the word “instead.” These spells have an upgraded effect if you control a creature with power 4 or greater as they resolve. For these, you only get the upgraded effect, not both effects.

Roar of Challenge doesn’t give any creatures the ability to block the target creature. It just forces those creatures that are already able to block the creature to do so.

Ferocious abilities of instants and sorceries that don’t use the word “instead” will provide an additional effect if you control a creature with power 4 or greater as they resolve.

If, during the declare blockers step, a creature is tapped or is affected by a spell or ability that says it can’t block, then it doesn’t block. If there’s a cost associated with having that creature block, its controller isn’t forced to pay that cost. If the player doesn’t, the creature doesn’t block in that case either.