R Code : (After bunzip2 t.csv.bz2)

z<-fread("~/q/m32/t.csv")
ulist = unique(z$grp)
timer=proc.time()
z2 <- z[,list(sumcol=sum(col3)), by='col1,col2,grp'][,as.list((sumcol[match(ulist,grp)])), by=c("col1","col2")]
proc.time() - timer
setnames(z2[],c("col1","col2",ulist))

   user  system elapsed 
 27.215   0.503  27.759 

Q Code: (this was used to create the t.csv file for use in R)
q)n:5000000
q)s:`a`b`c`d`e`f`g`h`i`j`k`l`m`n`o`p`q`r`s`t
q)\t exec s#(grp!col4) by col1:col1,col2:col2 from update col4:sum col3 by col1,col2,grp from t
11543
q)save `:t.csv
`:t.csv