Usually PE tubes have a life span of about 6 to 18 months, until the eardrum heals and eventually closes the opening where the tube is sitting and pushes it out into the ear canal. So, it is less common that the tubes have lasted only about 4 months, and that they need to be replaced that often. However, some kids with really bad ear infections that create a large amount of pus can actually pop out that tube with the sheer volume and pressure of the pus.
PE tubes are great because they reduce the number of ear infections and make it easier to treat the ones that do occur by using topical antibiotic drops instead of oral antibiotics. But, ear infections can still happen and it sounds like your child is one of those kids.
Some other ways to prevent or reduce the number of infections: get all those shots (flu, Prevnar), stop using a pacifier, don't bottle prop or allow baby to sleep with a bottle of milk, keep smokers away, reduce exposure to illness--some parents will actually consider stopping daycare to reduce the ear infections.