In Spanish we use past imperfect (aka copretérito) for many things, but these are the most relevant:
• When an action is not bound to a specific time limit: “Los niños jugaban todo el tiempo”, “antes solía visitar a mi abuela”.
• When two actions in the past occur simultaneously: “hacía frío cuando llegué”.
• When an action is not real (it may express whish, possibility or doubt): “Pensé que no te gustaba esta música”, “Me lo podías haber dicho antes”. “Tuve un sueño y mi casa no era mi casa”.
1 "estuve cansada...". Estuve = an action that is finished, hence pretérito.
2 "estaba trabajando..." "estaba trabajando" means an action that has finished BUT that you are describing with a gerund, so the action is not contained in a specific span of time. It remains "open". Therefore, the imperfect is the right tense to use.
3. "A las cuatro en punto..." in this case, you are defining the action according to another (main) span of time: 4.00 o' clock. So, everything that happens in this example is at the background (so to speak) of the main action which is to be (yes, simply to be) at 4:00 o' clock. Imperfect must be used when two things in the past occur simultaneously for the one in the "background".
I hope this helps. Do not hesitate to ask for more clarification.