四 means simply the number "4" of 12345 and 四つ is the counter for counting the object. Ee have original counter depends on the things and つ and こ are for the objects like fruits, vegetables, and a lot of things. Please imagine that there are 4 apples on the table and you have to write the number/letter "4 " on each apple. "4" of the four apples is 四つ, and "4" of "write the number 4" is 四. 一つ = 1つ、七つ = 7つ 1,2,3,4,5,6,7,8,9,10 --> in kanji, 一ニ三四五六七八九十 with the counter つ、ひとつ、ふたつ、みっつ、よっつ、いつつ、むっつ、ななつ、やっつ、ここのつ、とお *とお is used without つ but I don't know why. I hope this was helpful.