In v4.0, custom selects now have "width: 100%", causing them to fill up an entire line of a form. I assume that this is a bug, as it looks pretty weird. A perfect test case is the Bootstrap documentation. In the "Components > Forms" section, under "Inline forms", there is an example under "Custom form controls and selects are also supported" that includes a custom select. Back in v4 alpha 6, this select was only as wide as its widest text; now in the v4 full release, the select takes up the entire width of the form, pushing all following controls onto the next line, as seen below:

• v4 Alpha 6: https://v4-alpha.getbootstrap.com/components/forms/#inline-forms
  

• v4.0: https://getbootstrap.com/docs/4.0/components/forms/#inline-forms
  

I observed this behavior in Windows 10, on Chrome 63 (64-bit), Edge 41, and IE 11.