Is it reprehensible to repeat a prayer due to doubt about the number of rak'ahs without building upon the minimum or performing the prostration of forgetfulness?

You should have based your action on what you were certain of regarding the prayer—which is the lesser amount—then completed the prayer and performed the prostrations of forgetfulness (sujood as-sahw). This is because rectifying the prayer is better than repeating it, just as drawing closer to Allah with a mended prayer is better than turning away from mending it. And suffices oneself with it after mending is better than repeating it, because this was the methodology of the Prophet (peace be upon him) and his companions. However, the prayer that was repeated is valid.