How often are your builders busy?
If you can't keep 4 a majority of the time now, then don't get the fifth builder.
Use your gems on something else, finish that one last lab upgrade, but statues decorations that cost gems.
Why:Because if you can't keep four builders busy now, you'll never need the fifth builder more than likley, because you won't be able to keep it Busy, and if you can't keep it busy; whats the point in buying it? There is none.
If you always keep your 4 builders busy;Then get the fifth.
Why:You'll be thankful you have it. You'll constantly be using it. And your upgrades and base gets done faster.
So the answer is it depends if you can keep your builders busy.
Some people have five builders and it isn't enough.
Some people only have two and it is more than enough.