Did I mention weight, lipo's are much lighter and your (sons) B4 will fly, may need some lead strip under the battery for starters. Not seen much in the way of exploding lipos, have seen a few swelling due to mischarging, but generally I feel comfortable with our lipos, heres the 2010 approved list.
http://www.brca.org/BRCA/elecboard/n...20v2%20pdf.pdf
Find a local club and see what they are using, B4's still everywhere. if you are racing at clubs then you will need to match tyers, shocks, setups.