eAdd two fields in a column if their previous field values are same

Hi All,

I have two files

file1:

abc,def,ghi,5,jkl,mno
pqr,stu,ghi,10,vwx,xyz
cba,ust,ihg,4,cdu,oqw

file2:

ravi,def,kishore
ramu,ust,krishna
joseph,stu,mike

I need two output file as follows

If field3 in file1 is same as field3 in the next line then the field4 should add upto field4 in the next line and i should get a unique output as follows

ouput:

abc,def,ghi,15,jkl,mno
cba,ust,ihg,4,cdu,oqw

can somebody please help me with this as i require it pretty urgently. I am using sun solaris. If we can get the output using sed or awk that would be great. Any other way is also appreciated.

Thanks in advance..........

Double post, continued here