Interestingly, I heard an explanation about Prop 4 that has me wondering about it.
The amendment is not needed because the Constitution already prohibits and income tax.
Apparently Currently, the Texas Constitution requires voters to approve an individual income tax in a statewide referendum, which legislators can ask for with a simple majority in the House and Senate. Proposition 4 would raise the bar, amending the constitution so that any income tax resolution would need two-thirds support in both legislative chambers before the matter goes to voters, who would ultimately decide. And some people think that businesses would use the amendment to challenge any corporate tax now and in the future.
Senate Democrats raised objections about the measure's language, contending that using the term "individual" instead of "natural persons" - the phrase used throughout the state constitution's tax section - could precipitate legal challenges to the state's franchise tax (a tax Texas levies on businesses) and possibly exempt corporations and other business entities from paying up.