Log4j 1.x has reached end of life on August 5, 2015. So I would like to recommend you to use log4j 2.x which supports this with the monitorInterval configuration.
However according to the log4j 1.x docs here it does support log level changes runtime.
How can I change log behavior at runtime?
Log behavior can be set using configuration files which are parsed at
runtime. Using configuration files the programmer can define loggers
and set their levels.
The PropertyConfigurator defines a particular format of a
configuration file. See also the examples/Sort.java example and
associated configuration files.
Configuration files can be specified in XML. See log4j.dtd and
org.log4j.xml.DOMConfigurator for more details.
See the various Layout and Appender components for specific
configuration options.
In addition to configuration files, the user may disable all messages
belonging to a set of levels. See next item.