In my Surface Go 4 windows are opening at full screen

Have you tried checking the snap feature? You need to check that as well and turn it off. Go to Settings > System > Multitasking and find Snap windows and switch it off.

Can you change the scaling to 125% (for testing purposes) you can always go back and revert it to 150%(default). Once put on 125% scaling do your window test again see if there are any changes.

thank you for the additional information. Next, thing to do is check and reset folder windows settings.

Open File Explorer and restore it to a smaller size. There should be your (...) three dots, select it and go for Options. Once there check the View tab, click reset folders(if available). if available select it and click apply then restart your device.

Next, check these settings. Go to Settings > System > Display and tell me the details for:

Display resolution (for example, 1800 × 1200)

Scale (for example, 150%, 175%, 200%)

Can I ask few things when you created the new profile to test. 1. What window did you open and test, is it Control Panel windows, File Explorer windows, or all desktop applications?

1. When the windows opens check you mentioned it's on maximized, can you click the Restore Down button (the middle button between Minimize and Close), does the window shrink?
2. If you resize a window manually and close it, does it reopen maximized the next time?

This is a helpful test to determine if it's associated to a certain settings that might caused the problem. Go ahead and create a new user profile first and see if it shows the same Window behavior there.