Would mining cryptocurrency with a cluster of four Raspberry Pi 3's be any more efficient than just adding them to an existing mining pool and having them all mine separately? Would a Raspberry Pi cluster turn out a better hashrate?
tldr; No it wouldn't.
Most of the CPU only mineable coins have algorithms that demand a lot of RAM so that is a limitation on the Pi. Moreover from my experience mining if you use a pool or solo mine using RPC against a local daemon all your hashrates work towards the same block. So there is no need to cluster. Honestly it is better to use HC1 Odroids or SBCs with more RAM and CPU. Also the PI really needs heatsinks and a fan and DO NOT mine with all cores as it will shut down a lot. A bunch of Verium miners are solo mining using an RPI as the wallet daemon, I suggest reading the following Steemit post on how to do this to get a better understanding: https://steemit.com/verium/@birty/cpu-mining-is-back-a-complete-how-to-guide-and-profit-analysis-for-verium-mining-on-a-farm-of-single-board-computers-part-1
There's 0 point in using the RPi as a cryptocurrency mining rig. Clustered or not, there's no way the tiny processor and network IO has a chance in hell of competing with the specialized hardware that's being used by bigger players.
More to your question:
How a cluster would perform vs an individual RPi is nearly impossible to say. Different cryptocurrencies use different algorithms. How parallelizable your algorithm is will be very dependant on the cryptocurrency in question and how it's implemented. If it can be clustered, you'll have to either write code to cluster it, or find someone else who has already done the work.
Performance is hard to say. If it involves a lot of networked IO (even just between devices) your overhead will probably outweigh most of the cluster benefits.