Microstrip Wilkinson Power Divider
Your microstrip lines are shown as 0.348&quot; long, which is 8.84 mm. Given the speed of light in FR4 is roughly c/2 1, this corresponds to an electrical length of about 16 mm. As a Wilkinson is built with λ/4 line, it's tuned to a wavelength of about 64 mm, or a frequency of c/λ = 300e6 / 64e-3 = roughly 4.7 GHz, which is what your plot shows.
Double the length of the lines.
1 Very approximate, depends on geometry and frequency, especially with a horrible substrate like FR4, but it's good enough for gross back-of-envelope calculations like this.