How to include text just in one slide removing spacing using `beamer`

I am trying to remove space when using only in that particular slide, because it will produce a warning of overfull hbox.

Please consider this MWE:

documentclass{beamer}

usetheme{Copenhagen}

begin{document}

begin{frame}
begin{block}{}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut puruselit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabiturdictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna. Donec vehicula augue euneque. Pellentesque habitant morbi tristique senectus et netus etmalesuada fames ac turpis egestas. Mauris ut leo. Cras viverrametus rhoncus sem. Nulla et lectus vestibulum urna fringillaultrices. Phasellus eu tellus sit amet tortor gravida placerat.Integer sapien est, iaculis in, pretium quis, viverra ac, nunc.

begin{enumerate}[<+->]
    item
    begin{itemize}[<+->]
        item In the 4th slide, it should not include the following item space:
        begin{align*}
            a(x+1)^2-3xuncover<+->{&=ax^2+2ax+a-3x=ax^2+x+1\}
            only<4>{&implies ax^2+(2a-3)x+a=ax^2+x+1\}
            uncover<+->{&impliesleftlbracebegin{aligned}&a=a\&2a-3=1\&a=1end{aligned}right.}
        end{align*}
        item More text here.
    end{itemize}
end{enumerate}
end{block}
end{frame}
 
end{document}

Look at the end of the 4th slide:

After the big left brace, there is an extra blue space because of the following item More text here.

Is there any way to remove that extra space? The final output should look like:

I want solutions including the (probably) correct use of only, uncover, relative syntax etc.