Welcome to JOF
The 2009 19H (GVWR 4,950lbs, as you mentioned) will have a heavier "as shipped" UVW then the published UVW of 3,620lbs. The actual UVW (as shipped) should be noted on the yellow sticker on the outside of the HTT, or on a document attached to an inside cabinet door. I would assume the actual UVW will be close to 3,900lbs.
A moderately loaded 19H would be around 4,500lbs ((3,900lbs + 600lbs (empty tanks)), and with a recommended loaded tongue weight range of 13% to 15% = 585lbs to 675lbs. If you tow with water in the fresh water tank, your HTT weights will of course increase. Confirm the WD Mode weight limit on your 4Runners receiver hitch as well.
As mentioned, the loaded tongue weight is subtracted for the 4Runners payload capacity, as well as the 50lb WDH.
IMO your 4Runner will most likely be max'd out once loaded and hitched...., but that doesn't mean it's not doable....., just depends on your towing expectations. You stated the the 4Runner has a Tow Rating of 5,000lbs, please note that the "actual" tow rating may be less.
Actual Tow Rating: http://www.jaycoowners.com/forums/f37/what-is-my-actual-tow-rating-3866.html
I highly recommend that you visit a CAT scale once you have the TV/HTT under loaded conditions, that way you will know where you stand.
CAT scale how-to: http://www.jaycoowners.com/forums/f37/how-to-weigh-your-tv-tt-3871.html
Hopefully this information provides a little insight on what to watch out for.