4.
如图,抛物线\(y=ax^{2}+bx+3\)的图象过\(A(-4,0)\),\(B(1,0)\)两点,与\(y\)轴交于点\(C\),作直线\(AC\),动点\(P\)从点\(A\)出发,以每秒\( \dfrac{5}{4} \)个单位长度的速度沿\(AC\)向点\(C\)运动,运动时间为\(t\)秒,当点\(P\)与点\(C\)重合时停止运动.
\((1)\)求抛物线的表达式;
\((2)\)如图\(1\),当\(t=2\)时,求\(S_{\triangle BCP}\)的面积;
\((3)\)如图\(2\),过点\(P\)向\(x\)轴作垂线分别交\(x\)轴、抛物线于\(E\)、\(F\)两点.
\(①\)求\(PF\)的长度关于\(t\)的函数表达式,并求出\(PF\)的长度的最大值;
\(②\)连接\(CF\),将\(\triangle PCF\)沿\(CF\)折叠得到\(\triangle P′CF\),当\(t\)为何值时,四边形\(PFP′C\)是菱形?