If your goal is simply to maximize employer contributions, you should be fine. As you have observed, there wasn't a way to get fully matched on the 15k that you've already contributed, you were only matched on the first 4% of your salary. But, contributing more than 4% meant that you left none of that employer match money on the table, good job!
You say you have about $3,500 left to contribute for the year, and your new employer will match the first 2% of of salary deferrals. This means that you can still maximize the employer match without going over the personal contribution limit, as long as your new salary is $233,333 or less (assuming you started your new job April 1st or later). At this rate, you would earn $175,000 in the remainder of the year, and 2% of that is $3,500.
Say, for example your new salary is $100,000 per year. You would want to defer 3.5% of this to fill out the remainder of your personal contribution limit for 2018, if you had the whole year to do so. However, you only have 3/4 of the year left to make contributions, so really you need to contribute 3,500/75,000 = 4.6%. This is low enough to not exceed the personal limit by year-end and high enough to get the full company match of 2% on all remaining paychecks for the year.
Note: IRA contributions are not deductible against gross income when calculating your Massachusetts state income tax liability, so directing a higher portion of your earnings to a 401k there Vs. your Texas earnings wouldn't do you any favors from a tax perspective.
Edit: 401k deferrals are deductible against gross income for Massachusetts state income tax purposes.
Edit 2: If you want to make more deferrals at your new employer The consensus from other online fora regarding this topic (other people in basically the exact same situation as yourself) suggest that you would need to discuss this with the plan administrator at your previous employer. They should be able to distribute funds from your old 401k account and then report this as a returned "excess deferral" on Form 1099-R. This would count as taxable income to you, since it was not subject to payroll taxes at the time it was directed to your 401k.
Excess deferral, as the name implies, means you could only do this for the portion of your personal salary deferrals made that exceed the contribution limit of $18,500. So you would need to work with the administrator at the end of the year, when you know exactly what your total contributions have been between both 401k accounts.
It does not appear that the plan administrator is under any specific obligation to issue a distribution or file a 1099-R for you (in which case if you do go ahead and over-contribute then you'd have to work it out with the IRS) so plead nicely.
Note: depending on how Massachusetts handles your return, there might not be any benefit to going through all this. If you get a 1099-R for the returned excess deferrals then that amount will be taxable. I do not know if they would tax you on 3/4 of it (the portion of the year you were in MA), all of it, or none of it. Consulting a tax specialist might be a good idea.