A friend of mine has an older Ridgid hammerdrill / recip saw combo. Recently both of his 18 volt Nicad batteries took a dump and the tool are still performing strong after years of use on jobsites. He recently upgraded his batteries and charger to 2 of the 3.0 lithiums for the X3 line. Question, are the lithiums for the X3 line compatible with the new X4 hyper lithiums? Do all of the 18 volt batteries work with all of the 18 volt tool line, new or old, or do the X4 lithiums only work with the X4 tool line? I have an 18 volt cordless Ridgid hand planer kit (which performs great), but inevitably the time will come that I may need to replace the battery and would like to know my options since I've been eyeballing some of the X4 tools as well. Thanks for any clarification.