To extract the modal displacements for all modal forms, you need to iterate through each mode and retrieve the displacements individually. Here's an example of how you can modify your code to extract the modal displacements for all modes.
AnalysisResultsSetup.DeselectAllCasesAndCombosForOutput;
AnalysisResultsSetup.SetCaseSelectedForOutput('Modal');
PointName = 'C1-4';
ObjectElem = ETABSv1.eItemTypeElm.ObjectElm;
Obj = {''};
Elm = {''};
StepType = {''};
LoadCase = {''};
StepNum = 0;
NumberResults = 0;
U1 = 0;
U2 = 0;
U3 = 0;
R1 = 0;
R2 = 0;
R3 = 0;
% Get the number of modes
numModes = Results.ModeCount;
% Initialize the modal displacements matrix
modalDisplacements = zeros(9, 3, numModes);
% Iterate through each mode
for modeIndex = 1:numModes
[~, ~, NumberResults, ~, ~, ~, ~, U1, U2, U3, ~, ~, ~] = Results.ModeShape(PointName, ObjectElem, NumberResults, ...
Obj, Elm, LoadCase, StepType, StepNum, U1, U2, U3, R1, R2, R3);
% Store the modal displacements for the current mode
modalDisplacements(:, :, modeIndex) = [U1, U2, U3];
end
