t1 = linspace(start_time,end_time,number_of_samples)
For example, to use 100 samples over a one-second interval starting at t = 0:
t1 = linspace(0,1,100);
Then the rest can be like you have it already:
x1 = cos(2*pi*4*t1);
x2 = cos(2*pi*12*t1);
x3 = cos(2*pi*16*t1);
x4 = cos(2*pi*28*t1);
subplot(2,2,1); % select the 1st out of 4 subplots
plot(t1,x1); % plot a CT signal x1
grid on; % turn on the grid
xlabel('time (t)'); % Label the x-axis as time
ylabel('cos(2*pi*4*t)');% Label the y-axis
title('Graph of x1'); % Insert the title
subplot(2,2,2); % select the 2nd out of 4 subplots
plot(t1,x2); % plot a CT signal x2
grid on; % turn on the grid
xlabel('time (t)'); % Label the x-axis as time
ylabel('cos(2*pi*12*t)');% Label the y-axis
title('Graph of x2'); % Insert the title
subplot(2,2,3); % select the 3rd out of 4 subplots
plot(t1,x2); % plot a CT signal x3
grid on; % turn on the grid
xlabel('time (t)'); % Label the x-axis as time
ylabel('cos(2*pi*16*t)');% Label the y-axis
title('Graph of x3'); % Insert the title
subplot(2,2,4); % select the 4th out of 4 subplots
plot(t1,x2); % plot a CT signal x3
grid on; % turn on the grid
xlabel('time (t)'); % Label the x-axis as time
ylabel('cos(2*pi*28*t)');% Label the y-axis
title('Graph of x4'); % Insert the title
