There is only one file type that uses the .OP4 extension:
Nastran Output File
Output file created by MSC Nastran, a program that enables you to test the integrity of different structures such as buildings or spacecraft; generated by DMAP alters, which act like commands to extract information (like a matrix); can be written in Binary or ASCII format; contains vectors and matrices of that can be analyzed and output as a structure array by different Nastran readers.
The OP4 file is commonly used to exchange Craig Bampton models between different organizations for analysis. The model consists of large finite element models reduced into a set of small matrices (contained in the OP4 file) that contain mass, stiffness, and mode shape information which can then be connected to physical boundary points to test the low frequency response modes of the structure. The matrices are ideal for manipulation, which makes them easy to test in a wide range of analyses.
Updated: March 11, 2014