Bootstrap 3 Vs 4
LESS to SASS update in the case of CSS Source File in Bootstrap.
LESS is not as powerful as SASS. SASS comes with lot of functionalities like Nested Media Queries, Rem, Em, Mixing etc.
As far as learning is concerned, SASS is not that easy compared to LESS. Yes, time has come for you to get familiar with SASS.
The font size has been updated to Rem from Px.
Using the relative units of Px, Em and Rem are considered as smart choices when it comes to Typography. Rem has been utilized by Bootstrap 4. Rem is a more flexible choice compared to Px. Another good thing about Rem is that you can scale up and down elements without having to deal with a fixed size. This makes it easier for the developer to make the content device responsive.
Grid System Updated
Bootstrap 4 has 5 grid classes whereas Bootstrap 3 has only 4 grid classes.
The Xs from the lowest breakpoint has been removed in Bootstrap 4. That is not the case with Bootstrap 3. Now in the case of Bootstrap 4, it only relies on (.Col-) to deal with a lower breakpoint.
Flexbox Based Grid Included
Speaking of Bootstrap, it has introduced the crucial feature named Flexbox. This feature was not there in Bootstrap 3. It allows you to change the layout alignment in a quick manner. You can also change the size of the columns of the grid.
Tables are responsive
Thanks to Bootstrap 4, we can have fully responsive tables. You can make use of Table-Responsive class. It allows you to have horizontal scrolling of the tables, especially on smart phones. It works well on all sorts of screen sizes, whether it is mobile, tab or desktop.