If that's what you're picking up from the negative comments, then you're totally missing the point.
Letting your 4 year old use your smart phone is not going to screw them up. But giving them a cell phone *in lieu of* actually being there for them IS NOT going to be positive. Kids *should* be learning how to use the phone. How to remember Dad's work number (they should be able call from *any* phone, not just a single phone that happens to have your number programmed into it). They should be able to call grandma and explain how their day went, or what kind of excitement is planned for the day.
BUT a cell phone IS NOT a replacement for your real-life interaction. If the parent is gone so often that the primary care provider's phone is not enough, there is something seriously wrong. It really does fall into two categories... Either the parent is fostering a needy child where communication is expected to be immediate, at all times, and the parent is hovering. OR the parent is all but abandoning the child and to ease the quilt is throwing a video-capable cell phone at the 4 year old as a cheap parental replacement.
A balanced situation does not include the 4 year old *needing his own* phone.