1) Change the time signature to 6/4 from the following measure and onwards.

This time change now appears “a half measure too late”, but we will fix that later.

2) Use the plug-in to split the 4/4 measure:

Selection Tool.

Drag-select the measure partially, either the measure part before the time change, or the measure part after the time change (does not matter).

Plug-ins menu > Measures > Split Measure…

I suggest that you select the Barline Style {Dashed}.

You can always change the Barline Style when the time change is finished.

You might (or might not) prefer de-selecting the option {Move second part of split measure to next system}.

The plug-in automatically splits the measure into two “part-measures” where the second “part-measure” is not included in measure numbering.

In Other Words:

Now you have (technically speaking) two measures instead of one measure.

Hence you can change the time signature of the second “part-measure”.

3) Time Signature Tool.

Double-click the second “part-measure”.

In the dialog box Time Signature, extend the dialog box, to shows more options at the bottom.

Set the Measure Region to only that one measure.

I do not know how many beats you need in the “real” time signature (my guess is 3/4).

In the bottom of the dialog box you have the “display“ time signature set to 4/4.

Increase its Number of Beats to 6 so that the “display“ time signature is changed to 6/4.

4) You might like to change the Dashed Barline:

Measure Tool.

Double-click the first “part-measure” (= the one just before the time change).

In the Measure Attributes, in the upper pane, change the Style of the (Right) Barline.