Board size and the feel of a game
In tic tac toe the board is small and reads very quickly. The position becomes tense within the first few moves. In Connect Four the board is wider, which means a player has to keep more potential lines in mind. That makes a game longer and slightly harder for a beginner, but it also adds more room for planning.
Because of this, tic tac toe is excellent for fast series and short practice sessions, while Connect Four is more often experienced as a drawn-out game about controlling space.
The move mechanic changes the strategy
In tic tac toe you freely choose any empty cell. In Connect Four the disc drops down the column you pick. That is a very important difference. In the first game strategy is built on the pure choice of a point. In the second it comes from combining a column choice with an understanding of how the vertical drop shapes the future structure.
So Connect Four imposes more geometric constraints in a sense, while tic tac toe concentrates your attention more sharply on lines and strong cells.
Which is easier for a beginner
For a first encounter with line games, tic tac toe is almost always simpler. The rules are shorter, a game is faster, and the cause-and-effect link between a move and the result is more visible. Connect Four is easy to learn too, but there is more space and more variety, so the first games can be less transparent.
That is exactly why tic tac toe works so well as a base before longer games. It teaches the fundamental ideas quickly: blocking, control of the centre and the value of tempo.
Why the comparison is still worth making
The comparison shows that even a very simple game can be strategically rich. If Connect Four seems "more serious" purely because of its scale, tic tac toe is a reminder that depth can be born on a small board too. It simply shows itself faster and more sharply.
If you enjoy this kind of comparison, take a look at the piece on how it differs from gomoku and the general text on the history of the game.
Back to the short tactical game
If you want to feel what makes a compact grid duel good, the easiest thing is to open a few quick games right in your browser.