For the most part, the 3.4oz rule isn't strictly enforced, in the sense that they don't normally physically check ever single container you have to make sure that it's less than 3.4oz. If it's significantly larger than 3.4oz they will normally be able to see that on the X-ray, and will physically inspect it.
As an example, I travel with a can of spray deodorant that is around 4oz, but is only labeled with "100g", where other cans of the same product are labeled as "4oz (100g)". To date, having gone through airport security probably over 100 times (incl 5 times in the past 3 days) I've had them manually inspect it exactly twice.
So if what you're traveling with is close to 3.4oz, you're probably fine. However, if they decide to look at it, and it's over 3.4oz, then you'll almost certainly lose it. It's up to you whether it's worth the risk/cost of losing whatever it is you're trying to take through. If it's a 4oz tube of toothpaste it's a different answer than if it's a 5oz bottle of $200 perfume!