Easiest recipe will be isotonic salt and soda. It is the least irritating to the colonic mucosa as the osmotic pressure is pretty well balanced and the pH is slightly alkaline (if you want to get really involved you can use one of those PEG+electrolyte colon prep solutions to balance everything, but the added benefits are marginal - isotonic salt+soda doesn't show any visible irritation in the study I saw).
Best position will be one for you that combines relaxing the most with opening up the colon. This may be knee-chest (supporting parts of your body on pillows can help here, also put a pad under your knees), on your side, on your back, whatever. Standing puts more pressure and fullness on the rectum/anus, but if you can relax the best that way then it might make up for the water column in the descending colon. On days when my colon is not cooperating but I'm being an idiot and insisting on draining the enema I find that "rolling" into different positions can help. You don't want to do something where you need to tense your abs, but gentle movement can help the water distribute itself.
That said I would strongly encourage you to not press for a fixed number, whether it's 4 quarts or, in my case, 2.5-3. Take the enema until your body tells you its had enough, then stop. You can take a rest, roll to a new position, take a few deep breaths and do some gentle abdominal massage. After that if you feel you must click the clamp open again, but when your body tells you you've had enough go ahead and stop (maybe do the relax, roll and massage once more). Not everyone's colon will take a 4 quart enema, and not everyone's colon will take the same amount every time. Sometimes 1.5 quarts is just fine for me, sometimes I need the 2.5-3 or the water gets "stuck". Sometimes it just isn't working no matter what. Listen to your body and things usually work better.