I don't think all four states would be admitted at once, the western half of Texas would've been to sparsely populated to do so.
The could admit a two eastern states (maybe being divided by the Brazos or Colorado Rivers) on the get go. But you would need two northern states to balance it. However after Texas in OTL the next five states to be admitted (Iowa, Wisconsin, California, Minnesota, Oregon) were all free states. So the balancing issue might come around until Kansas comes along (which was still a problem in OTL), unless another west Texas state is created before. Then yes you would need one more northern state. Maybe a "East Dakota" scenario.
Also southerners will have 2 more senate seats. So the more interesting part of this is how it will affect the rising tensions in the next 15 years.