How many does RS4 have?
The amount of lines says nothing. Things like comments or longer scripting methods can add many lines without making a script better (or worse).
RS1 contained about 11k lines. RS2 contained 20k lines and was unplayable due to it being advanced beyond SA:MP at the time.
After a major rewrite, RS2 became RS3 with only 10k lines of code.
However after splitting it for use by multiple scripters it was back to about 15k at the start of RS4.
Since then a lot has been added.
But from the rewrite from RS2 to RS3 (in 1.5 months) a lot was learned in efficiency and memory use. With most of the functionality intact the memory use decreased with 40%, and the number of lines by 50%. Main problem for the 'next generation' was that there were zero comments....

Edit:
RS5 is new in many ways. We have tried to determine the down sides of RS4 and find a way to work around it. That includes redesigning the data structure to make things not only faster and more secure, but also more flexible.
Like RS2/3 the concepts of RS5 are holding new surprises never seen before.
This is why stress testing will be important, as what works with 15 testers may not work with 100 players on server (found that out in RS2).
I think we can be very proud of the team that worked the past 5 years on RS3 to RS4.1 (CBFASI, Gimli, Haldir, David_Omid, Louis_Keyl, iDavid and not to forget Conroy - hope I did not forget anyone) and managed to turn my code in to something readable without making it too buggy.
RS5 is made in a new way and is new and exciting in its design as well as features. Regardless if we will bring over the old stuff or not, it is going to last for a long time in to the future.