Being a fellow lefty and having owned both a '78 4001 and now a new 4003 I feel I am in a perfect position to advise! I see you have already decided on a 4003 but I will tell you what I think anyway.
As you have already gathered, both are excellent basses in their own right, which unfortunately makes the choice between the 2 that much more difficult.
Without doubt my 4003 is the better of the 2 basses in relation to construction and quality, but the neck is a lot chunkier than my 4001, so if you have small hands you may need to consider this, Ric necks have varied in size over the years but my '78 was quite slim.
Even with the vintage push pull switch and despite what people tell you the 4003 can't quite get the clanky growl of the 4001 (possibly due to the higher output pickups) but could also be down to the thicker neck as well, so if you are looking for what I would call the 'classic' Ric sound then the 4001 is the one to go for.
Personally I found the 4001 to be a bit lighter but then the 4003 isn't excessively heavy anyway.
'Unofficially' you have to be careful as to what gauge strings you use on a 4001 with it having a weaker neck, however I never experienced any issues with mine.
The newer 4003's have skinnier horns (which I prefer to the thicker horns of the 4003), but this is all down to personal preference.
To summarise, I really miss my 4001, in particular it's tone and skinny neck, but am very happy with my 4003 which has a more versatile sound. The ideal scenario is to get both of course, but for now I think you are probably wise in going for the 4003.