A "cost that contains " may be a spell's total cost, an activated ability's cost, a suspend cost, or a cost you're asked to pay as part of the resolution of a spell or ability (such as Broken Ambitions). A spell's total cost includes either its mana cost (printed in the upper right corner) or its alternative cost (such as suspend), as well as any additional costs (such as kicker). Basically, if it's something you can spend mana on, it's a cost. If that cost includes the mana symbol in it, you can spend mana generated by Rosheen on that cost.

You can spend mana generated by Rosheen on any part of a cost that contains . You're not limited to spending it only on the part.

You can spend mana generated by Rosheen on a cost that includes even if you've chosen an X of 0, or the card specifies that you can spend only colored mana on X. (You'll have to spend Rosheen's mana on a different part of that cost, of course.)

You don't have to spend all four mana on the same cost.