ASA 4 is, for example, a septic patient. Is your patient as sick as one of those? If yes, ASA 4, if no, ASA 3.
For example, a poorly-controlled hypervolemic/acidemic/hyperkalemic etc. ESRD patient would be a 4. A ketoacidotic DM would be a 4. A hypertensive crisis would be a 4.
There must be something that is ongoing (i.e. constant) and could result in death soon, if untreated (i.e. threat to life).