Can we use the modules which have been introduced in Splunk 5 for 4.3? Will it work at all??
If they have been specifically introduced in v5 then no, the previous version would have no idea if they existed or not.
If we're talking Advanced XML then append /modules to the end of your server URL. e.g. hostname:8000/modules to see a list of modules you can use within your instance to check.
Also, converttodrilldownsearch was available in 4.3. If you mean the simple xml drilldown funkyness that came along in v5 then no, you won't be in luck. Time to upgrade!