From the ASM Heat Treater's Guide
SAE 4140 Steel Tempering
Tempering
Temperature Hardness
Brinell Hardness
Rockwell Tensile
400F 200C 514 HB 55 HRc 297 KSI
500F 260C 477 HB 50 HRc 243 KSI
550F 290C 461 HB 48 HRc 235 KSI
600F 320C 444 HB 47 HRc 225 KSI
650F 340C 429 HB 46 HRc 217 KSI
700F 370C 415 HB 44 HRc 210 KSI
800F 430C 363 HB 39 HRc 182 KSI
900F 480C 331 HB 36 HRc 166 KSI
1000F 540C 293 HB 31 HRc 145 KSI
Data extrapolated from a hardness graph given in Brinell with nearest points taken from a conversion table for Rockwell hardness. Temperature conversion to Celcius rounded to nearest 10. Tensile not specifically for 4140. All data +/- 10%
