Where has .btn-default gone in Bootstrap 4?

The btn-outline-secondary class in Bootstrap 4 generates the closest to what used to be btn-default in Bootstrap 3.

That seems to be "typo" there in the docs.

primary is the blue button/color and secondary is the grey ones.

And the very light grey is btn-light.

Here's the reference link for Bootstrap 4 buttons:

https://getbootstrap.com/docs/4.0/components/buttons/

add this css to your stylesheet file problem gone

.btn-default {
    color: #333;
    background-color: #fff;
    border-color: #ccc;
}
.btn-default:focus {
    color: #333;
    background-color: #e6e6e6;
    border-color: #8c8c8c;
}
.btn-default:hover {
    color: #333;
    background-color: #e6e6e6;
    border-color: #adadad;
}
.btn-default:active {
    color: #333;
    background-color: #e6e6e6;
    border-color: #adadad;
}