What type of radioactive decay results in uranium-238 becoming thorium-234?

The atomic number of U is 92 and for Th it's 90

From 238 to 234 is a change in 4 of mass number.

92 to 90 means 2 protons were lost.

An alpha particle consists of 2 protons with a mass number of 4.

That would be an alpha decay.