You cannot set position of axes in tiledlayout, so I would recommend setting the Layout,Tile property of the fourth axes after it has been created. I'd also avoid reusing the same variable name to capture your axes. Also, use nexttile when appropriate.
Here's an example
%% Plot ISA Troposhere
format long
%% US Customary Units
% Atmosphere
alt=0:36089/100:36089; %BG
p=2116.2166237*(1-alt./145442.16).^5.2558797; %psf
T=59-3.562e-3*alt; %degF
rho=p./(1716.372*(T+459.67))*32.17; %lbm/ft3
Tc=(T-32)*5/9;%degC (for mu).
mu=1.458e-6*sqrt(Tc+273.15)./(1+110.4./(Tc+273.15))*0.020885; %pa-s to psf-s
% Plot p vs alt
figure1=figure;
figure1.Position=[500 200 600 700];
t = tiledlayout(figure1,1,2);
ax1 = nexttile;
plot(ax1,p,alt,'color',[0,0.45,0.74])
ax1.XColor = [0,0.45,0.74];
ax1.YColor = [0,0.45,0.74];
xlabel('Pressure, p [psf]');
ylabel('Altitude, h [ft]');
ax1.YLim=[0,36089];
ax1.GridColor=[0.1,0.1,0.1];
y1ticks=ax1.YTick;
for i=1:numel(y1ticks)
y1tickfixed{i}=num2str(y1ticks(i),'%.0f');
end
set(ax1,'XGrid','on','XMinorGrid','on','YGrid','on','YMinorGrid','on');
ax1.YTickLabel=y1tickfixed;
% Plot T vs alt
ax2 = axes(t);
plot(ax2,T,alt,'-k')
ax2.XAxisLocation = 'top';
ax2.YAxisLocation = 'right';
ax2.Color = 'none';
ax1.Box = 'off';
ax2.Box = 'off';
xlabel('Temperature, T [^{o}F]');
ax2.YLim=[0,36089];
ax2.GridColor=[0.1,0.1,0.1];
y2ticks=ax2.YTick;
for i=1:numel(y2ticks)
y2tickfixed{i}=num2str(y2ticks(i),'%.0f');
end
ax2.YTickLabel=y2tickfixed;
set(ax1,'XGrid','on','XMinorGrid','on','YGrid','on','YMinorGrid','on');
% Plot rho vs alt
ax3 = nexttile;
plot(ax3,rho,alt,'color',[0,0.45,0.74])
ax3.XColor = [0,0.45,0.74];
ax3.YColor = [0,0.45,0.74];
xlabel('Density, \rho [lbm/ft^3]');
ylabel('Altitude, h [ft]');
ax3.YLim=[0,36089];
ax3.GridColor=[0.1,0.1,0.1];
y1ticks=ax3.YTick;
for i=1:numel(y1ticks)
y1tickfixed{i}=num2str(y1ticks(i),'%.0f');
end
set(ax3,'XGrid','on','XMinorGrid','on','YGrid','on','YMinorGrid','on');
ax3.YTickLabel=y1tickfixed;
%Plot mu vs alt
ax4 = axes(t) ;% WHAT DO I PUT HERE, THIS JUST STICKS IT BACK IN TILE 1?
ax4.Layout.Tile = 2;
plot(ax4,alt,mu,'-k')
ax4.XAxisLocation = 'top';
ax4.YAxisLocation = 'right';
ax4.Color = 'none';
ax3.Box = 'off';
ax4.Box = 'off';
xlabel('Dyn. Viscosity, \mu [psf-s]');
