>>11193682>the SRBs produce 70% of the thrust.
Only for liftoff to T+ 2:00 or so, which is the point in flight where Isp matters least.
For the rest of the ~9 minute burn to orbit, the core stage will suffer from gravity losses with only 4 engines and a low TWR for pretty much the entire burn. Adding a fifth engine increases dry mass by reduced gravity losses to the point that, as >>11193722
mentioned, the performance gains would be significant. That payload to LEO increase also directly corresponds to an increased delta V to yeet the upper stage with even if it's only carrying a light payload, because as I mentioned, total core delta V goes down but the actually useful fraction of delta V goes up.