So now you have 4 wrong estimates to work with :) To have some predictability you should have small features. That's the only thing that can give you strong estimates. No industry has solved giving strong and correct estimates - it's in the name, it's estimation!

Yes small features help enormously. I've used ROPE and work breakdown structures (WBS) to estimate multiple projects at thousands of staff hours to under 4% of actual. The ROPE advantage is that it uses multiple perspectives, which gets stakeholders understanding that the estimates are imperfect and depend on many factors.