Flexbox: 4 items per row

You’ve got flex-wrap: wrap on the container. That’s good, because it overrides the default value, which is nowrap (source). This is the reason items don’t wrap to form a grid in some cases.

In this case, the main problem is flex-grow: 1 on the flex items.

The flex-grow property doesn’t actually size flex items. Its task is to distribute free space in the container (source). So no matter how small the screen size, each item will receive a proportional part of the free space on the line.

More specifically, there are eight flex items in your container. With flex-grow: 1, each one receives 1/8 of the free space on the line. Since there’s no content in your items, they can shrink to zero width and will never wrap.

The solution is to define a width on the items. Try this:

.parent {
  display: flex;
  flex-wrap: wrap;
}

.child {
  flex: 1 0 21%; /* explanation below */
  margin: 5px;
  height: 100px;
  background-color: blue;
}

<div class="parent">
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
  <div class="child"></div>
</div>

With flex-grow: 1 defined in the flex shorthand, there’s no need for flex-basis to be 25%, which would actually result in three items per row due to the margins.

Since flex-grow will consume free space on the row, flex-basis only needs to be large enough to enforce a wrap. In this case, with flex-basis: 21%, there’s plenty of space for the margins, but never enough space for a fifth item.