StringJoin with Spaces

In M10.1+ you can use StringRiffle:

StringRiffle[
    {"hi","how","are","you"},
    " "
] //InputForm

"hi how are you"

Another option is to use ListFormat

  TextString[list, ListFormat -> {"", "    ", ""}]

gives

   "hi    how    are    you"

The Map function will "do the same thing" to every element in a list. So we use that to StringJoin a space onto the end of every word in your list, except the last word. And then we Use StringJoin on that plus the last word in your list to put all those together into a single string. So

list = {"hi", "how", "are", "you"};
StringJoin[Map[StringJoin[#, " "]&, Most[list]], Last[list]]

which gives you the string "hi how are you"