Hi, I know this may be a bit late but worth a note.
First off, MES has a pka of 6.15 and a formula mass of 195.24g/mol. At any pH +/-1 of the pka that buffer is no longer a buffer and is effectively useless. So, maybe try another buffer if you wish to have a pH around 4 (maybe a phosphate buffer idk what is safe for plants).
*Edit* I got your working and starting pH confused... I feel like this is still worth noting though
On another note, since you added 1g/L MES this roughly translates to 5mmol/L MES (1g/195.24g/mol = 0.00512mol/L) which is again, such a low concentration the buffering capacity is no longer existent. Common buffering start concentrations are usually around 50mmol/L or 10x your current working conc.
Finally, when working with buffers you will always need to adjust your pH. The whole theory of buffers is that they contain a conjugate acid and a conjugate base. And when you buy a buffer it is either in the fully acidic form, or fully basic form. Mixed buffers may work, but I would still pH them since the ratios may change.