Can I use both the LATCH system and the vehicle seat belt at the same time?
No, you must not use both the vehicle seat belt and LATCH belts at the same time when using the car seat rear- or forward-facing. Both installation methods are equally safe, but only one should be used at a time. Choose either the vehicle seat belt or the LATCH system for installation.
From the official Graco Graco 4ever Dlx Usermanual manual, page 8
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