We are currently using Spring 3.2.7.RELEASE as of the last upgrade ticketed at:
TRUNK-3390, a brief exploration of the available upgrade options suggests several such as 4.2.0 snapshot, 4.1.4 current, 4.0.9 and 3.2.13 but we would most likely opt for 4.1.4. as announced at: https://spring.io/blog/2014/12/30/spring-framework-4-1-4-4-0-9-3-2-13-released
And here is the release notes page
An upgrade from Spring Framework 3.2 (or earlier) to 4.1.4 should be as straightforward as a change of version numbers in the Maven POM files. Note that you may have to upgrade specific third-party dependencies to a recent enough version (e.g. Hibernate 3.6+, Jackson 1.8+ for an upgrade to 4.0.1), but other than that, you should be able to get all the benefits of the Spring 4 line with hopefully very little effort.
Code is being committed to https://github.com/openmrs/openmrs-core/tree/spring-4 and tested by https://ci.openmrs.org/browse/TRUNK-MASTER0