My boys enjoy using the computer for their school work when they get the opportunity, and for my 4th Grader, Timez Attack is just the answer!
There are "skins" available to change the game setting, and they do add some special fun to the game. The free version uses a dungeon setting and that might be just fine for your student.
Personally, I recommend the "Rental" of the Standard game, unless you have younger children that you plan to use it with, in which case go for the "Buy" version so you can use it forever.
It's $25 for the year to rent the standard edition, but after a year, your child will be a whiz at Math facts! You can also use it for more than one child concurrently. We tried the free version, but one thing for sure, kids love variety and so for me, it was well worth the cost for the extra levels. And, the Retention feature is more fully developed in the rental version, from a teaching perspective, that's always a bonus.
This game is highly entertaining, and in our house you can frequently find another child watching over the shoulder of the student who is playing. This has the side benefit of having older children refresh their facts or giving younger children a "leg up" on math facts for when their turn comes!