To be on safer side try to use xfa.resolveNode() feature. For ex:
xfa.resolveNode("form1.#subform[0].Table4.Row2[1].hours1").rawValue = xfa.resolveNode("form1.#subform[0].Table4.Row2[0].hr0").rawValue + xfa.resolveNode("form1.#subform[0].Table4.Row2[1].hours").rawValue
Thanks,
Bibhu.