Swim -How FAR???

If it's in a pool, you should definitely train over-distance. Open water is much more difficult. Really try to get that other open water swim in, and go slightly over the distance then if you can (say, 600 yd or more b/c weather/water conditions on race day can be more difficult).

Try to build your pool swim up to 750, at least. Four weeks is also plenty of time to work on your stroke (absolutely get a coach or a very advanced swimmer to help you, if possible, and as soon as possible so you can practice the next few weeks).

 Check out the 0-1650 plan (http://ruthkazez.com/SwimWorkouts/ZeroTo1mile.html).   Many of us have had great success with it.  And think, by the time your tri gets here you'll already be swimming a mile in the pool.  That'll give you great confidence on the 500yd swim.  

Todd

I used to train by just swimming whatever distance the race was, and adding to it. So if if was a 500 yd swim, I'd swim 750.

Now I have been concentrating more on swimming drills and a continuous long swim once a week and have alot more considerable gains than the previous method.

If a swim is 500 yards long, I know I am better trained for that by swimming 5 x100 yd sprints, than always going out and only swimming 500 yds.

Exactly what I would say.  You are better served doing most of your swims as shorter sets focusing on really good form then to just go out and swim continuously.

I've swam about 1/2 less this year than I did last year, but with more focused and consistent work I knocked 2 minutes off of my IM swim time.  Once again, on a lot less yardage.

And if it is an open water swim then you can throw any sort of accurate measurement out the door ...... OWS are notoriously mis-measured.