Can the ADL5201 operates for frequencies under 4 MHz? If I change the output chokes by resistors, will it work at low frequencies?
No, the internal feedback capacitors limit low frequency operation to 4 MHz.
I see. Is there an equivalent of this amplifier for lower frequency range (with digitally controlled gain, power down control)?
How low in frequency do you wish to operate? The devices I suggest would be the AD8369, AD8370 and depending on how low in frequency, the AD8375.
While looking at the datasheets of your recommendations I noticed the AD8366 which is a Dual DC-600 MHz VGA that would fit exactly our needs. Do you recommand it?
Yes, If the AD8366 will meet your requirements then you should go ahead and use it.