4 columns to 2 columns

this is the error: the col25p class is misplaced.

.col-25p .responsive #top #wrap_all .flex_column.avia-link-column.av_one_fourth

The css means cascading style sheet
The ID/class which is highest in the DOM tree is named first – then the following ones.
Your class: col25p is definitely not before .responsive – because that is a class in html.
Since I don’t see your page, you have to find out yourself where in the structure your class was set – probably if you set it to the column it is :

@media only screen and (max-width:767px) {
	.responsive #top #wrap_all .flex_column.av_one_fourth.col-25p {
	width:49%!important;
	float: left !important;
	}
	.av_one_fourth.col-25p {
		margin-left:1%!important;
	}
}

ist that really what you want?: https://webers-testseite.de/4cols/ ( top columns)

but i guess you want to influence all columns in that color-section – it might be better to place the custom-class to the color-section
and redifine the columns width of all cols in it.

custom class on color-section: col50

@media only screen and (max-width:767px) {
	.responsive #top #wrap_all .avia-section.col50 .flex_column.av_one_fourth {
	width: 48% !important;
	float: left !important;
	}
	.avia-section.col50 .av_one_fourth:nth-of-type(2n) {
		margin-left:3% !important;
	}
}

see green columns on test page