Editor's note: This story has been updated to include the University of Texas at Dallas among the universities given the "Carnegie Tier One" status.

Four Texas universities — Texas Tech University, the University of North Texas, the University of Texas at Arlington and the University of Texas at Dallas — have reached a major milestone in their quests to join the top tier of the nation's colleges.

The Carnegie Classification of Institutions of Higher Education listed the universities among 115 schools designated in its highest ranking for research activity. The designation, often referred to as "Carnegie Tier One," was previously held only by four Texas schools — the University of Texas at Austin, Texas A&M University, Rice University and the University of Houston.

The rankings focus solely on research prominence. But the designation is a sign that the universities are moving closer to state leaders' goal of increasing the number of overall "tier one" universities in Texas, university and state leaders said.

"It is quite an accomplishment," said Raymund Paredes, commissioner of the Texas Higher Education Coordinating Board. "It is obviously an indication that our emerging research universities are in fact emerging and becoming top research universities."

There is no universally accepted standard for what overall "tier one" means, but in general, schools are expected to bring in at least $100 million per year in research grants, plus have selective admissions and high-quality faculty. Another common measuring stick is membership in the prestigious, invitation-only American Association of Universities. Currently, UT-Austin, A&M and Rice are widely considered to be tier one. The University of Houston also frequently claims that designation based on its top Carnegie ranking, though it isn't a member of AAU.

The universities that moved up in the research rankings celebrated the recognition Tuesday. Tech Interim President John Opperman said the ranking was the "culmination of years of dedication to research, teaching and learning by our administration, faculty and students." UT-Arlington credited its strategic hiring and investment. UNT called it a major step in its drive for national prominence.

"This achievement reflects our commitment to excellence in our education and research mission and the quality of our students and graduates," UNT President Neal Smatresk said. "Today's recognition is an important step in our journey − but it's not the end."

Disclosure: Texas A&M University, Texas Tech University, The University of Texas at Arlington and The University of Texas at Austin are corporate sponsors of The Texas Tribune. The University of North Texas was a sponsor in 2014. Rice University and the University of Houston were sponsors in 2013. A complete list of Tribune donors and sponsors can be viewed here.