Basic strength order of (1). aniline 2). 2,6-dimethyl-aniline (3). 1,1-dimethyl aniline (4). 2,6,1,1-dimethyl aniline are?

Ortho effect (SIP) in ortho Substituted aniline:

Ortho effect in case of ortho substituted aniline is also known as steric Inhibition of protanation (SIP).

In ortho substituted aniline after protanation due to crowding  of ortho substituted groups bond strain of H-N-H bond increases and molecule becomes unstable so we can conclude that ortho substituted aniline avoid to protanation hence it's basic strength also decrease than aniline.