I have not solved that puzzle yet. Your idea of the gammon killing four cube is a candidate. Consider these other rollouts.
  White is Player 2
score: 0 pip: 159  4 point match  pip: 167 score: 0
Blue is Player 1  
XGID=bECeEbdbB:0:0:1:21:0:0:0:4:10 
Blue to play 21 
1.  Rollout^{1}  13/11 6/5  eq: 0.2324 
 Player: Opponent:  46.26% (G:12.56% B:0.92%) 53.74% (G:18.74% B:2.75%)  Conf.: ±0.0047 (0.2371...0.2277)  [100.0%] Duration: 4 hours 32 minutes 

2.  Rollout^{1}  24/23 13/11  eq: 0.2481 (0.0157) 
 Player: Opponent:  45.85% (G:12.38% B:0.90%) 54.15% (G:17.92% B:1.47%)  Conf.: ±0.0042 (0.2523...0.2439)  [0.0%] Duration: 4 hours 20 minutes 


^{1} 31104 Games rolled with Variance Reduction. Dice Seed: 71940290 Moves and cube decisions: 3ply

eXtreme Gammon Version: 2.10, MET: RockwellKazaross
  White is Player 2
score: 0 pip: 159  5 point match  pip: 167 score: 0
Blue is Player 1  
XGID=bECeEbdbB:0:0:1:21:0:0:0:5:10 
Blue to play 21 
1.  Rollout^{1}  24/23 13/11  eq: 0.2061 
 Player: Opponent:  45.96% (G:12.40% B:0.63%) 54.04% (G:17.95% B:0.93%)  Conf.: ±0.0030 (0.2092...0.2031)  [90.0%] Duration: 3 hours 42 minutes 

2.  Rollout^{1}  13/11 6/5  eq: 0.2091 (0.0030) 
 Player: Opponent:  46.33% (G:12.58% B:0.66%) 53.67% (G:18.67% B:1.67%)  Conf.: ±0.0034 (0.2125...0.2057)  [10.0%] Duration: 4 hours 00 minute 


^{1} 31104 Games rolled with Variance Reduction. Dice Seed: 8281536 Moves and cube decisions: 3ply

eXtreme Gammon Version: 2.10, MET: RockwellKazaross