Well, first you get the annoying lecture
: If you are building a new system the power supply is the LAST thing you want to skimp on. You should buy a new, modern PS for the system.
Now that's out of the way.
There is nothing technically wrong with how you want to do it. The power supply delivers 12v to each yellow wire regardless of where they actually go. So if the MB needs four 12v wires (and 4 black grounds) to run you can accomplished that your way.
There could be a problem - resulting in odd behaviors and system problems - if the 12v leads came from different 12v rails on a multi-rail power supply. This could result in "dirty power", where the amperage (as opposed to voltage) is not the same across all 4 wires at the same time. This could even happen in a single rail system, depending on the internal circuitry of the PS and where those two separate wire bundles connect.
The only thing that concerns me from your picture is that 3 of the yellow wires are in the back row and that one (plus the extra one hanging off) is in the front row. This seems to be a problem with polarity and I am surprised the system runs like that. I think that all 4 yellow wires should be in all the rear slots.