A filter designed at 1kHz sampling rate, when used at 4kHz sampling rate,
its frequency response will be stratched 4 times. For example the new
cut-off frequency will be 4 times the original.
If you want to keep similar frequency responce as the original, you need to
redesign the filter. Normally you have to use more cofficients in the new
design in order to keep the same shape.
Use the filter design tool: "fdatool", it will teach teach you how to
design filters.
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