As you seem to have remembered in some places, but not others, you need ".^" to do element-wise raising to a power. Correcting this error in two places gives:
x4 = 0:pi/30:2*pi; % x vector, 0 <= x <= 2*pi, increments of pi/30
x4a=(137.*267.^(1/2).*111819872067690549716262153787.^(1/2).*1i.*(71289 - 2500.*x4.^2).^(1/2))./785298692348313600 - (748846716930964699.*x4)./15705973846966272;
y4a=-sqrt(5.34^2-x4a.^2);
plot(x4,x4a)
That code runs, but gives a warning about ignoring imaginary parts of your complex input. Maybe you can sort that out yourself?
