I need ref design for LTC3536 Vin 3V-4V2 and Vout 5V@700mA, DS is unclear about frequency and inductor choice.

Can you comment on why you are looking for a buck-boost converter for this application?  Typically buck-boost converters are used in applications where the input voltage varies above and below the regulated output voltage.  In your application it appears the output voltage is always above the input voltage.  Is that true?  If it is true, perhaps a boost converter would be appropriate for this application.  There are several boost converters available with a variety of features (synchronous, output disconnect, etc.)  The LTC3122 is one part that might meet your requirements.  The boost converter selection guide available on the analog.com web site can also suggest other parts.   

If you would like to evaluate the LTC3536 for your application you can look at the demo board (DC1852A).  Reducing the lower feedback resistor, R2, from 220k to 137k Ohms will give a 5V output.  The demo board should provide a good platform for evaluation.

Can you comment on why you are looking for a buck-boost converter for this application?  Typically buck-boost converters are used in applications where the input voltage varies above and below the regulated output voltage.  In your application it appears the output voltage is always above the input voltage.  Is that true?  If it is true, perhaps a boost converter would be appropriate for this application.  There are several boost converters available with a variety of features (synchronous, output disconnect, etc.)  The LTC3122 is one part that might meet your requirements.  The boost converter selection guide available on the analog.com web site can also suggest other parts.   

If you would like to evaluate the LTC3536 for your application you can look at the demo board (DC1852A).  Reducing the lower feedback resistor, R2, from 220k to 137k Ohms will give a 5V output.  The demo board should provide a good platform for evaluation.