4 in. x 10 ft. 220-PSI PVC Sch. 40 DWV Plain End Pipe

yes

Frankly, I've had no problem of that sort using slip-by-slip couplings with both pipes fully engaged. However, if you want the bell-and-spigot engagement you described, look at Home Depot bell end pipe offerings: Unfortunately, all these are rated Sewer and Drain (schedule 10 wall thickness). To insist on schedule 40 in that configuration, search for PVC SCHEDULE 40 BELL END PIPE. You'll find products by GF Harvel, Cantex, Charlotte and others - sometimes available in 20 foot lengths. These are not sold at my local Home Depot, but you may be more fortunate. Another alternative is to use "4" PVC Sch. 40 Inside Connector (Pipe I.D. Spigot x Pipe I.D. Spigot)" fittings. [This is precisely the type of fitting you described.] Each of these is priced about the same as another length of pipe.

they do sell them in I think its four foot pieces.

Schedule 40 refers to the pipe wall thickness which varies in conformance with stress ratings based on pipe diameter. Thus all pipe IDs depart slightly from their nominal values. You can see various IDs and ODs at sources such as https://www.pennusa.com/products/schedule_40.php. In the present case: 4.026 inches

Bed the pipe properly. Back-fill with sub-base material compatible with that extant. Bond your replacement floor slab to that surrounding the area / trench. Do not cover the pipe with an isolated patch - not bearing on anything else. You should be OK. I wouldn't place a column supporting the main girder of my house directly over this pipe - but I have buried such pipe in my landscaping and driven over it with a pick-up truck.

Schedule 40 fittings are compatible. The schedule number refers to wall thickness. Those values being the same ensures compatibility between pipe and fittings in each respective diameter.

Realistically, neither. Pipe lengths do not arrive in pristine condition and surfaces bear a combination of original process finish, imprinting and handling and exposure effects. PVC is rather simple to prep and paint if that is your intent. An important consideration for such painting, however, is UV resistance. There are clear coats made to impart such resistance to more common base colors.

yes it is.

UV is known to discolor PVC. See https://www.heritageplastics.com/technical-bulletins/effect-sunlight-exposure-pvc-pipe-conduit-fittings/. There are methods of painting PVC to mitigate this effect.