Just to clear up something....
The Regulations (Part 61) state the following wrt ATPL requirements:
61.07.2___An applicant for the issuing of a airline transport pilot licence (aeroplane) shall have completed not less than 1 500 hours of flight time, of which_–
(a) 250 hours shall be as pilot-in-command or not less than 100 hours as pilot-in-command and 150 hours as co-pilot performing, under the supervision of the pilot-in-command, the duties and functions of a pilot-in-command;
(b) 200 hours shall be cross-country flight time, of which not less than 100 hours shall be as pilot-in-command or as co-pilot performing, under the supervision of the pilot-in-command, the duties and functions of a pilot-in-command;
(c) 75 hours shall be instrument time, of which not more than 30 hours may be instrument ground time;
(d) 100 hours shall be night flight time as pilot-in-command or as co-pilot; and
(e) not more than 100 hours may be acquired in a simulator.
This has to be read together with:
Crediting of flight time 61.01.16_
(3)__A commercial pilot or airline transport pilot shall be entitled to be credited with the total flight time during which he or she acted as pilot-in-command.
(4)__A commercial pilot, when acting as co-pilot in an aircraft normally required to be operated with a co-pilot, shall be entitled to be credited with not more than 50 per cent of the total flight time during which he or she acted as such, towards the total flight time required for a higher grade pilot licence.
What (4) actually means is that of your total requirement of 1500 hrs for ATPL, only 750 hrs may be co-pilot hours. Thus, even though you might have 4000 co-pilot hours on an a/c required to be flown with a co-pilot (remember 4000/2=2000), only 750 of the 2000 will be considered towards the next higthest license.
Therefore, if you have a total of 4500 hours (4000 P2), the other 750 hours (of the min requirement) must be P1 and/or as per breakdown in 61.07.2. This is where most of the SAA cadets fall out of the bus, because they come back with a frozen ATPL, go into the RHS of a J41/DHC8/ERJ/CRJ, clock the P2 hours, but do not get any additional P1 hours to satisfy the minimum requirement.
If it is still confusing - join the club....