Purchase Order Form needs incrementing numbers for three separate departments

I am new to macros and vb. I am pretty good with Excel's front side, formulas, and so on. It is automating actions to minimize mistakes and work time that I need some help in.

I cannot post my file because I am new and have no permissions to post, so the best I can do is explain it like this:

I need to take the numbers from k2:n2, k3:n3, or k4:n4 depending on which department is chosen, and combine them into a single string that looks like this 16-17-001-00 in cell B4, so that I can tell the third set of numbers to increment. Or I need a macro that will increment only the data from the third set of numbers in the string if it is typed as a whole in any one cell.

I have set up buttons to attach macros to for the different departments so that when the correct button is pressed the department numbers p.o. base will load and increment by one each time.
Our company p.o. looks like this 16-17-001-00 and the only part that increments is the 001 section. It represents our fiscal year, the p.o. number, then the department code and it is absolutely not something I can alter, it has been long established as the standard form. We have three department codes 00, 02, and 03.


Please help if you can and thanks in advance.

Ok, I have tried to attach the file, but last time I tried it resulted in a php format for some reason