What is the best way to secure safely a 10 oz cake topper on the top of 4 tier cake? Thanks for your suggestions.

What does the topper look like? What is it made out of?

If the weight of it is distributed evenly all over the base of the topper (i.e. it doesn't weigh more in the back than the front, for example), I would put 4 drinking straws into your top tier under where the topper will go, and secure the topper using royal icing as a glue. The straws are to stop it sinking or you could use dowels if you prefer.

It's actually like adding another small tier, think I'll go with dowels. Thanks Dayti for your advice.