Kevin Federline's defence team have subpoenaed the rehab clinic where his ex-wife Britney Spears was a patient to testify in their custody battle. Kevin's attorney Mark Vincent Kaplan confirmed "an administrative executive" at Malibu's Promises centre - where the 25-year-old troubled singer checked into earlier this year after shaving off her hair - was served legal papers on Saturday. The unnamed staff member is the fourth person to be subpoenaed to give evidence about Britney's parenting of her sons, Sean Preston, 23 months, and Jayden James, 11 months. Britney's former assistant Shannon Funk, her bodyguard Daimon Shippen and her cousin Alli Sims have also been targeted by Kevin's legal team. Promises declined to comment on the subpoena, citing the facility's privacy policy. Earlier this month, Kevin, 29, filed for primary custody of the couple's two sons, which would entitle him to have Sean and Jayden in his care for 70 per cent of the time. Custody is currently shared equally between Kevin and Britney.Meanwhile, Kevin's former lover Shar Jackson - who has a five-year-old daughter, Kori, and a three-year-old son, Kaleb, with the backing dancer - has defended his parenting skills. Speaking about the impending custody battle, she said: "Kevin is a great dad!"Shar's support marks a drastic change of heart. The former 'Moesha' star hasn't always been so confident in his fathering abilities. She once said: "I told Kevin, 'I'm not going to let you be a convenient father, when it's convenient for you.'"If you are going to be part of these kids' lives, then you're going to be a part. If not, then you're not.' "