Unfortunately, it is not possible to update 32 fields across four channels in one command. You can use four commands with the same syntax that you show above to update four channels. I would put a small delay of 10-100 ms between each call to avoid a burden on the server. Dont forget to use the correct API key for each channel.
If you absolutely have to update all 32 values in a single call, it is possible to pack multiple values into one field, but then you will need to unpack them again later. For example, you could write
...?api_key=xxxxxxxxxxxxxxxx&field1=value1,value2,value3...value32
Then when you read the values from ThingSpeak, you will need to parse the date for the comma. Also, in this mode the standard field plots wont show your data, but you could write a MATLAB visualization that reads the values and parses them, and then displays them.
