Why Camellia has 4 Sbox
The only way you will have a certain answer will be by mailing their designers.

Using multiple S-boxes increases the size of the code and decreases the possibility of working with different blocs in parallel.

Using a single S-box also simplify the way the specifications have to be written. The simpler the specifications are, the less prone to make error at the implementation step you are.

Not related to Camellia but on the same idea:


  JP Aumasson: why nobody uses SHA-3? FIPS 202, a 29-page specs written for cryptographers, not for users [source]
  
  JP Aumasson: seriously NIST, in 2.5 years no one had the idea that, perhaps, it would be good to simplify that thing? like, ditch the greek part [source]


However, using a single S-box implies some sort of symmetry in your bloc cipher which MIGHT be the reason why the designers of Camellia preferred to use 4 S-boxes instead of only one.