That valve your mechanic described as very, very tight was probably the majority of your problem. As the engine warmed up the valve no longer sealed properly so you probably had the fourth cylinder dropping out.

Injectors do not usually go downhill as soon as the engine warms. So what was wrong is more dependant on what he did first.

If he adjusted the valves and engine still ran marginal when warmed a little it probably needed the injectors. My bet is he did the injectors first and the engine was still rough as soon as it warmed a little. Then he adjusted the valves and the engine cleared up.

It really pays to do your own service on your cars when possible. In your case the injectors may not have been in good shape. If they have been substantially improved you should now see better fuel milage than you had before this problem occured. If fuel milage is identical then the injectors might have been just fine. .