In 2/4, the first beat of each measure receives the stress.

1 - 2 | 1 - 2 | 1 - 2 | etc.

In 4/4, the first beat of each measure receives the main stress and the third beat is also stressed, but not as strongly as the first beat.

1 - 2 - 3 - 4 | 1 - 2 - 3 - 4 | etc.

The difference can be quite subtle.

Don Firth