Extracting information from lines having columns occuring more than once - awk

Question

I have a file :
chr1 1234 2345 EG1234:E1
chr1 2350 2673 EG1234:E2
chr1 2673 2700 EG1234:E2
chr1 2700 2780 EG1234:E2
chr2 5672 5700 EG2345:E1
chr2 5705 5890 EG2345:E2
chr2 6000 6010 EG2345:E3
chr2 6010 6020 EG2345:E3
As you can see there is a specific ID before ':' and there is an id that is repeated after ':' which might be common to more than one row , I want an output that looks something like this:
chr1 1234 2345 EG1234:E1 (output as it is since it doesn't have duplicate id in the next row)
chr1 2350 2780 EG1234:E2 (since duplicate the 1st and 2nd column of 1st occurrence &
3rd and 4 th column of the last occurence)
similarly
chr2 5672 5700 EG2345:E1
chr2 5705 5890 EG2345:E2
chr2 6000 6020 EG2345:E3
I was trying to use a key to move to next column but I am not quiet sure as to how would I extract the column wise values
awk '{key=$4; if (!(key in data)) c[++n]=key; data[key]=$0} END{for (i=1; i<=n; i++) print data[c[i]]}' file1
In short I want to extract the first two columns of first occurrence and last two columns from the last occurrence of any rows with duplicate 4 th column

Answer 1

This one only messes up the record order:
($1 FS $4 in a) { # combination of $1 and $4 is the key
split(a[$1 FS $4],b) # split to get the old $2
a[$1 FS $4]=b[1] FS b[2] FS $3 FS b[4] # update $3
next
}
{
a[$1 FS $4]=$0 # new key found
}
END {
for(i in a) # print them all
print a[i]
}
Test it:
$ awk -f foo.awk foo.txt
chr1 EG1234:E2 2350 2780
chr2 EG2345:E1 5672 5700
chr2 EG2345:E2 5705 5890
chr2 EG2345:E3 6000 6020
chr1 EG1234:E1 1234 2345
One-liner:
$ awk '($1 FS $4 in a) {split(a[$1 FS $4],b); a[$1 FS $4]=b[1] FS b[2] FS $3 FS b[4]; next} {a[$1 FS $4]=$0} END {for(i in a) print a[i]}' foo.txt

Answer 2

Using awk, considering the key1:key2 as a unique combination and if applying it to filter duplicates. Here $4 represents the key1:key2 from your file.
awk '!seen[$4]++' file
chr1 1234 2345 EG1234:E1
chr1 2350 2673 EG1234:E2
chr2 5672 5700 EG2345:E1
chr2 5705 5890 EG2345:E2
chr2 6000 6010 EG2345:E3
The logic is straight forward, the line identified by key1:key2 is printed only if it is not seen already.