awk help

[jaisask]

i am newbie in awk and bioinformatics.

i have a tab deliminated text file. the format is as follows. 3 records, 4 fields separated by tabs
SNP 100 101 102 103
M1 a:5 b:4 a:26 b:3
M2 a:4 b:45 a:6 b:18
M3 a:50 b:40 a:26 b:30

please help me in filtering the file based on number behind the colon [:].

1) let's say, if the number after the colon [:] equal or more than 5, then just print "a" or "b"
2)if the number after the colon [:] is less than 5 then print "X".


Thanks in advance