Can I use both the LATCH system and the vehicle seat belt at the same time to install the car seat?
No, you should not use both the LATCH system and the vehicle seat belt simultaneously when the car seat is in 5-point harness mode. You must choose one or the other for installation. Always consult your vehicle owner's manual for specific installation requirements. (Page 7)
From the official Graco 4Ever DLX Car Seat manual, page 7
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