It's hard for me to see any human playing 4/1 here, but that is what XG 3ply normal plays.
  White is Player 1
score: 6 pip: 145  7 point match Crawford  pip: 131 score: 5
Blue is Player 2  
XGID=bABBBBBbacbcbCA:0:0:1:21:5:6:1:7:10 
Blue to play 21 
1.  3ply  4/1  eq: +0.666 
 Player: Opponent:  49.72% (G:33.56% B:1.03%) 50.28% (G:16.95% B:0.70%)  

2.  3ply  3/2 3/1  eq: +0.610 (0.055) 
 Player: Opponent:  49.55% (G:30.97% B:0.77%) 50.45% (G:14.83% B:0.44%)  

3.  2ply  24/23 3/1  eq: +0.608 (0.058) 
 Player: Opponent:  48.34% (G:32.04% B:1.10%) 51.66% (G:17.53% B:0.69%)  

4.  2ply  24/22 2/1  eq: +0.492 (0.174) 
 Player: Opponent:  45.73% (G:28.87% B:1.40%) 54.27% (G:18.01% B:0.54%)  

5.  1ply  6/5 6/4  eq: +0.305 (0.361) 
 Player: Opponent:  41.68% (G:23.56% B:0.79%) 58.32% (G:18.05% B:0.72%)  

eXtreme Gammon Version: 2.19.208.prerelease, MET: Kazaross XG2
After forcing a 3ply evaluation of the top 3 candidates:
  White is Player 1
score: 6 pip: 145  7 point match Crawford  pip: 131 score: 5
Blue is Player 2  
XGID=bABBBBBbacbcbCA:0:0:1:21:5:6:1:7:10 
Blue to play 21 
1.  3ply  24/23 3/1  eq: +0.811 
 Player: Opponent:  54.07% (G:36.49% B:1.25%) 45.93% (G:14.63% B:0.57%)  

2.  3ply  4/1  eq: +0.666 (0.145) 
 Player: Opponent:  49.72% (G:33.56% B:1.03%) 50.28% (G:16.95% B:0.70%)  

3.  3ply  3/2 3/1  eq: +0.610 (0.201) 
 Player: Opponent:  49.55% (G:30.97% B:0.77%) 50.45% (G:14.83% B:0.44%)  

eXtreme Gammon Version: 2.19.208.prerelease, MET: Kazaross XG2
Rollout:
  White is Player 1
score: 6 pip: 145  7 point match Crawford  pip: 131 score: 5
Blue is Player 2  
XGID=bABBBBBbacbcbCA:0:0:1:21:5:6:1:7:10 
Blue to play 21 
1.  Rollout^{1}  24/23 3/1  eq: +0.877 
 Player: Opponent:  56.91% (G:36.94% B:3.79%) 43.09% (G:12.34% B:1.51%)  Conf.: ± 0.009 (+0.868...+0.886)  [100.0%] Duration: 8 minutes 47 seconds 

2.  Rollout^{1}  4/1  eq: +0.636 (0.241) 
 Player: Opponent:  49.76% (G:32.04% B:2.41%) 50.24% (G:15.29% B:1.43%)  Conf.: ± 0.009 (+0.627...+0.645)  [0.0%] Duration: 8 minutes 55 seconds 

3.  Rollout^{1}  3/2 3/1  eq: +0.626 (0.251) 
 Player: Opponent:  50.58% (G:30.72% B:3.50%) 49.42% (G:12.85% B:1.20%)  Conf.: ± 0.008 (+0.618...+0.634)  [0.0%] Duration: 8 minutes 50 seconds 


^{1} 1296 Games rolled with Variance Reduction. Dice Seed: 271828 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.19.208.prerelease, MET: Kazaross XG2