delat or wye and ground current

[cleveland]

We have an installation of numerous transformers on a large 12470 solidly grounded system. This system comes from the utility company. Most of the transformers (~97%)have delta primaries. New voices are stating that new units installed should have wye primaries. The major driver for this is the fact that the local utility only stocks wye type transformers themselves and in an emergency we'd use one of theirs. I'm a bit conflicted here as I have always considered the 4th wire in our primary system to be a ground. A utility company engineer once told me that whether this 4th wire is considered a ground or a neutral depends on how you used it, i.e. if you were putting currents onto the 4th wire it was a neutral but if you weren't putting current on it, it was a ground. If we used wye primary transformers aren't we putting the unbalanced cuttent in the bank or the 3 phase unit right on the 4th wire? But by using delta primary aren't the unbalanced currents resolved by circulating currents within the bank or the unit? I'm confused as you can tell but it goes aginst my past to just start putting wye primary units in all over.... any insight would be appreciated.