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abcd=4(dcba)
1000a+100b+10c+d=4(1000d+100c+10b+a)
1000a+100b+10c+d=4000d+400c+40b+4a
996a+60b=3999d+390c
332a+20b=1333d+130c
↓ ↓ ↓ ↓
8 7 2 1
a=8; b=7: d=2; c=1
Luego:
a+b+c+d=8+7+1+2=18