The cake features the four main characters from the franchise coming up from through a man-hole cover, with her son's name written in multi-coloured icing. The city skyline is printed around the edge of the cake, with Skittles pouring out from a sewage pipe.
The tweet read : "Had such a special day- celebrating this sweet little one's (soon-to-be) birthday!!! #Tennessee (sic)"
Another picture posted alongside the image of the cake featured Tennessee stood outside their house holding a balloon with a grin on his face.
Text written on the photo read "Tennessee turns 4! (sic)".
Tennessee is the youngest of Reese's children, and the only child she shares with husband, talent agent Jim Toth.
The 'Wild' star's other children, 17-year-old Ava and 12-year-old Deacon, come from her previous marriage to actor Ryan Phillippe.
Ava took to her own Instagram account to post a picture of her brother and wish him a happy birthday.
She wrote: "happy early birthday to this super preschooler going on teenager. I love you, dude. (sic)"
And Tennessee isn't the only one to get a special birthday message from his mum, as the 'Legally Blonde' actress also posted on Instagram to celebrate Ava's birthday earlier this month.
Alongside a picture of her look-a-like daughter, she wrote: "Can't believe my baby is 17 today! I love you, @avaphillippe Your gorgeous heart and loving spirit never cease to amaze me! I'm so lucky the universe let me be your mother. (sic)"
