Neo - the answer to your question "which is better accumulation or waterfall" fundamentally
depends on your long-term objectives or goals. Are you trying to get to Financial Independence
as quickly as possible or are you trying to reach a specific net worth/wealth goal. If your primary focus
is to FIRE as quickly as possible, the numbers lean more towards waterfall as the best choice. Here
is my rationale: You currently have 3 properties that are accumulating $2100 in net profit ($25.2K per year).
If you went ahead and bought that 4th property, your monthly net profit would be $2800 ($33.6K per year).
You stated with the waterfall method, you would be able to pay off each property free and clear every 2 years.
Then it would only take 8 years to pay off all 4 properties free and clear. With no mortgage in 8 years, your total
net profit would jump to around $4400 ($52.8K per year). This is assuming a jump of $400 for each b/c you have
no mortgage. $52.8K per year in passive income = FIRE. Just to make sure we are on the same page, I consider net profit
as money left after mortgage, insurance, property tax, property mgmt, repair estimate, vacancy estimate, and Cap Ex.