@Bob B.
Just spoke with my agent. If you have $150k in coverage you are insured up to that amount regardless if it is a partial loss or total loss. Surcharges are added to your premium based on the number of units. The more units the higher your premium. These surcharges are meant to account for properties with multiple units. So no matter what happens you are covered up to the insured amount. In this case $150k.
Now if the building catches fire and 3 of the 4 units are destroyed I would be covered up to $150k. If the cost to rebuild those 3 units is more than $150k then I would gladly take my 20% profit (remember I bought the property for $125k) sell the burned property as is, and walk away with a nice return on my money.
Side note: this coverage also covers up to $15k in lost rent which is also adjustable based on your preference.
BiggerPockets actually did a podcast on this subject so it surprises me there is so much difference of opinion on this topic.