Dynamic html tag loop creation

arr = [1,2,3,4]
str = ''
i = 0
arr.each do |a|
        if i % 3 == 0
                str<<"#{a}"
        else
                str<<"#{a}"
        end
        i = i+1
        if i % 3 == 0 || arr.length == i
             str <<"
"
        end
end

puts str.inspect

output is like :

1
2
3
4