We do advise immersing on time when your husband is on base if there is even a small chance that he could get unexpected leave.
We are not familiar with a general leniency to permit reducing the five day minimum to four days when a husband is in the army, but if a specific situation comes up (particularly if there is urgency, such as trying to conceive, etc.) be sure to ask an individual question.
There may be additional leniencies available (for example, minimizing the onot perishah you must observe) but these leniencies are applied on a case by case basis. Be in touch with us if a particular situation comes up.
Unfortunately it can be very challenging when a woman's niddah cycle does not always work out with her husband's army schedule. If you are currently using hormonal contraceptives, you may manipulate your cycle to work around your husband's schedule (as best as you can).
Please feel free to get back to us with any further questions.
B'hatzlacha!