MSCRM+ Is it Mandatory!!!!!!!

Hello All,

Is it always compulsory to use the crm sdk w.r.t. MSCRM version.

Say

for crm 4.0 SDK also 4.0.

for crm 2011 sdk also 2011

for crm 2013 sdk also 2013

for crm 2015 sdk also 2015

for crm 2016 sdk also 2016.

If so , then which are the respective links to get the crm sdk for

Versions 4.0,

               2011,

 2013,

 2015, and

2016

Thanks in Advance.