In my Surface Go 4 windows are opening at full screen

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 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.

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.