data1 = "AAA.txt"
data2 = "BBB.txt"
data3 = "CCC.txt"
data4 = "DDD.txt"
plot data2 u 2:4 w l lt 1 lw 3.5 lc rgb "red" title "b-O", \
data3 u 2:4 w l lt 1 lw 3.5 lc rgb "blue" title "c-O", \
data1 u 2:4 w l lt 1 lw 3.5 lc rgb "web-green" title "a-O", \