BFR TKOF, a PLT of XA-KUT was briefed that the Houston Hobby (HOU) WX was IFR with '. . . indefinite ceiling zero sky obscured visibility's 1/16 of a mile and fog . . .' He was also adzd IFR conds were forecast until 0900 CST, gradually improving to marginal VFR by 1100 CST with 1000' brkn, 5 mi vis & fog. The FLT tkof off at 0900 CST. At HOU, arrival was delayed due to WX. The crew requested an ILS apch, with intentions of diverting if a missed apch was made. The RVR was variable. After being clrd for an ILS RWY 4 apch, the plt was adzd the RVR had dropped to 1400'. After changing to TWR freq, the FLT was clrd to lnd & was told again the RVR was 1400'. When the ACFT was on final apch, the plts were adzd of a low alt alert. However, the ACFT contd blo the decision height (DH), hit a 70' pwr line (aprx 6500' short of the RWY) & crashed. No preimpact malfunction of the ACFT or its systems was fnd. The ILS was flt checked, but only a minor discrepancy (concerning the outer marker width) was noted. The DH was 200' AGL, min RVR for the apch was 2400%.