If your walls currently have uneven levels of paint, applying a new layer won't fix the problem. The different levels will still show through the new paint. You'll have a solid, uniform wall color, but the surface won't be smooth. However, you can perform some prep work or use special paints to help with uneven walls. Both methods are successful, but each has a drawback. Prep-work is very time consuming, but it's low cost. Specialized paint is quick and easy, but it will cost more to paint a room.
Apply plaster patching compound to very deeply recessed layers of paint. If the paint layers are uneven by a margin of less than 1/4-inch, you can sand the layers smooth without plastering.
Smooth out the wet plaster using a putty knife. Wait for the plaster to dry.
Sand the uneven wall surfaces with 150-grit sandpaper. Sand directly on the protruding patches to remove the paint until it is even with nearby layers. Essentially, you want to buff out the topmost layers of uneven paint, leaving only the bottom layers intact. Sandpaper also smooths out plaster patches for a better-looking finish. For 400-square feet of wall space, this could take several hours with an electric sander, and much longer if done by hand.
Wipe down the wall with a moist rag to removed the sandpaper dust. Don't over-saturate the rag; just get it barely damp enough to remove the dust.
Tape any trim or other surfaces with painter's tape.
Apply interior latex paint to the wall surfaces. Use a roller for the large wall surfaces, and a brush for the tight corners and edging. When the first coat dries, you can apply a second coat if necessary.
Remove the painter's tape when the paint has fully dried.
Protect the edges of the room with painter's tape.
Apply elastomeric latex paint to the uneven wall surfaces. Use a roller for main surfaces, and a brush for edge-work. Elastomeric wall coatings (EWCs) are formulated to be very thick, yet pliable. EWCs are traditionally used for mason-work to fill in gaps during painting, but they work equally well for covering uneven wall levels. Elastomeric paint will cost you more though, as this thick paint provides less coverage per gallon. You'll likely go through several extra cans of paint when compared to a regular latex paint. If you're more concerned with the project's time than its cost, EWCs eliminate the need for prep work, and they also cover in just one coat.
Remove the painter's tape once the elastomeric latex paint has dried.