Depending of your aim you could use subconfluent cell layer or usual population in log phase.
So density could be same for every type of plates or flasks. If you succesfuly used 4000 cell/wel in 24 well plate then density was 4000 cell / 1.9 cm2 (area depends of suplyer) = 2105 cell/cm2
To obtain same density in T-175 multyply density to area: 2105 ceel/cm2 x 175 cm2 = 368421 cell per flask.
There are a number of handbooks available online from varoius companies like invitrogen and thermo. These may help you to get a basic idea of seeding density of cells in tissue culture flasks or plates with the different surface diameter.