Rihanna and A$AP Rocky celebrated their son RZA’s 4th birthday with a blowout bash in New York City. The proud parents threw the fun fête at The Sloomoo Institute in Soho, Page Six confirmed. A source told us that the “Diamonds” singer and rapper “rented out the entirety” of the popular slime museum for the evening. Rihanna and A$AP Rocky threw RZA a 4th birthday party at the Sloomoo Institute in Soho Wednesday night (seen above). Jairo / BACKGRID Rihanna was seen entering the downtown venue with one of her sons (seen above). ASPN / BACKGRID We’re also told that the party was “filled with their family and friends”— including their two other kids Riot, 2, and 8-month-old Rocki — but there were no additional A-listers in attendance.…