new byte[4096] every time — simple, and 4 MB of garbage per thousand calls.

[Benchmark (Baseline)]
public int New_array()
{
    var sum = 0;
    for (var i = 0; i < 1_000; i++)
    {
        var buf = new byte[4096];
        buf[0] = (byte)i;
        sum += buf[0];
    }
    return sum;
}

ArrayPool rent/return — reuse instead of garbage.

[Benchmark]
public int ArrayPool_rent()
{
    var pool = ArrayPool<byte>.Shared;
    var sum = 0;
    for (var i = 0; i < 1_000; i++)
    {
        var buf = pool.Rent(4096);
        buf[0] = (byte)i;
        sum += buf[0];
        pool.Return(buf);
    }
    return sum;
}

stackalloc inside a helper method — the frame (and the buffer) is reclaimed on every return. Never stackalloc directly in a loop: the stack only unwinds at method exit.

[Benchmark]
public int Stackalloc_span()
{
    var sum = 0;
    for (var i = 0; i < 1_000; i++) sum += UseScratch(i);
    return sum;
}