Answer
$$1$$
Work Step by Step
Consider: $S_n=a_1-a_2+......+(-1)^{n+1}a_n$
and $$|S-S_n| \leq |a_{n+1}| \\ \implies |\space Error |=|S-S_4| \leq |a_{4+1}|=|a_5| \\ \implies |(-1)^4 \times t^5| \\ \implies t^5 \lt 1$$
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