How to buy next property using property as collateral

[Jorge Guadian]

 I would like to hear  from experienced investors or people that have used there collateral to leverage there money on investment properties. I have four duplexes two of them are paid off  those two are bring in $2925 a month in rents.  Estimated worth 150K the other 130K.. The other two I have mortgages on after the mortgage payment bring in $1430 month in rents.  Each one is estimated at 100K. I moved my butt this year and made it a goal to  purchased 2  duplexes this year. One was purchased in April and the other one in October. There is not much equity  in those properties.  Im looking for ideas on how to structure a way to buy an other investment property with minimal money  using  my collateral with the duplexes that are paid off. If anyone has done  a purchase using there collateral Im all ears, all suggestions welcomed.

Thank you

Jorge