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What is the check-in deadline for your KLM flight?
When traveling on separate tickets, it's often wise to build in a lot of extra time for the connection. If your first flight is delayed or cancelled and you miss the second flight, you might be forced to buy a very expensive walk-up fare for a subsequent flight (or cancel your trip entirely).
If there are delays with the bus transfer, you can always take a taxi from T8 to T4.