Generic Space Marine Army 4000 Points................

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Here's the problem, you don't have a lot of stuff to represent lots of Companies. With what you do have however, this is what I would do.
Veteran Company (18x Veterans, 1x HQ, 10x Rhino)......850 points
Dreadnought Detachment (Any type or combination)......100 points
Thunderhawk...........................................100 points
Thunderhawk...........................................100 points
Razorback Detachment (6x Razorbacks - replace 3x Rhino)..400 points
Close Combat Terminators (4x Termi + 2x Land Raiders)...300 points
Warhound Scout Titan Pack (2x Warhound Titans)..........250 points
(both with 2x Vulcan Mega Bolters each).......200 points
Battle Company...................................850 points
(6x Assault, Tactical + Devastators + HQ and 10 Rhinos)
Bike Detachment (5x Bikes)..............................150 points
Bike Detachment (5x Bikes)..............................150 points
Land Speeder Squadron (5x any type).....................200 points
Whirlwind Artillery
OR Hunter AA Battery (3x)....150 points
Vindicators (3x Vindicators)............................150 points
Apothecary OR Chaplain.................................50/75 points*
(*Chaplain will take you to 4025 but it's within the legal limit.)
There, that at least is what you can field and should have something for everyone.