DECIMAL(38,4) means a decimal number with up to 38 digits in total, of which 4 digits can be after the decimal point, one can adjust the precision if needed.
Please see the example of primitive Kafka Schema definitions and their corresponding payloads:
Transforms
Schema Transforms Supported Types
Mapping Kafka Data Types and SQL Types.
Very simple example of a JSON payload:
And the corresponding schema:
To make it easier to use the Kafka types, please find below the list of supported types mapped to usual SQL types that might be found in source database.