Hi Oliver,
3.70 refers to the Informix Client version not the JDBC compliance/standard version. Informix JDBC 3.70 driver is JDBC 3.0 compliant but not 4.0. If you need to use the JDBC 4.0 compliant APIs, you can consider using the IBM Data Server driver for JDBC and SQLJ (JCC).
cheers,
Pradeep.