Yep, it's quite common using 4 instead of Ч. But I think they do this cause the word четыре starts with Ч. And in different languages this Ч sound is spelled differently and it's rarely written down in one commom Latin letter without waves, stresses and so on above it (the only suitable language I can think of right now is Italian).
BTW, I see it often that people use 6 instead of Ш. For the same reasons I guess.