It sounds like A615 rebar.  If so, the C.E. does need to be determined so that the minimum preheat temperature can be established in accordance with the minimum preheat and interpass temperature table in D1.4.  Unlike A706, The C.E. of A615 rebar is not controlled and must be determined via the chemical analysis, which is typically included on the MTR .  The C.E. of ASTM A615 rebar generally falls between 0.6 and 0.7, so preheat is almost always required.  On the other hand, the C.E. of A706 is controlled to provide a carbon equivalent not to exceed 0.55%. Therefore no preheat is required for commonly used bar sizes up to 3/4” (#6).  Edit: (no preheat is required for these commonly used sizes unless the base metal is below 32°).  I thought it wasn't really necessary to mention that, as it's not an issue during this time of year.  The C.E. can be calculated as:

C.E. = %C + %Mn/6 + %CU/40 + %Ni/20 + %Cr/10 – %Mo/50 – %V/10

You’d mentioned direct butt splice between new and existing #4 bars, and a WPS for testing their welders to qualify for any rebar welding that may come along. 

The limitation of variables for WPS qualification are:

6.2.1.1 The largest rebar size number to be production welded must be used for WPS qualification.  Qualification for each specification and grade of rebar is not required, provided the WPS parameters are within the essential variables as listed in Table 6.1.

6.2.1.2 The WPS qualification must be performed using a steel that has a C.E. at least equal to the highest value C.E. to be encountered in production.  The WPS is then qualified for the highest C.E. value qualified, and all lower values.

6.2.1.3 Also, a change from uncoated bare to coated bars or steel where the coating lies within 2” of weld joint preparation, or any greater distance required to prevent the coating from melting and contaminating the weld metal will require requalification.

That being said, there isn’t a one WPS for all rebar to come along, that I know of.   

Before going further, I have the 2011 edition.  There were changes made from the previous release, so they’ll be more changes than what your 2005 edition indicates.

For welder qualification, the limitation of variables are:

6.3.2.1 Qualification with a particular process and with any of the steels permitted by D1.4 are considered as qualification to any other approved steels with that process except for galvanized steel.

6.3.2.2 The smallest bar size number used in qualification qualifies the welder for welding that bar size number and any larger size.

6.3.2.3 The base metal used must comply with 1.31 or the WPS.  The base metal; may be coated or uncoated, or galvanized, as required by 6.3.2.1

6.3.2.4 A welder qualified with an approved electrode and shielding medium combination is considered qualified to weld with any other approved electrode and shielding medium combination in accordance with Table 5.1 for the process used in the qualification test.

6.3.2.5 A change in the position of welding to one for which the welder is not qualified requires qualification in that position.

You’d mentioned a direct butt splice.  The direct butt joint groove weld test made with the SMAW, GMAW, or FCAW processes will qualify the welder for CJP groove welds and fillet welds. 

6.3.3.2 Item 1: The test assembly will be a CJP groove weld made by SMAW, GMAW, or FCAW using the smallest bar size number used in fabrication.

Positions of test welds and related validity of qualification for direct butt joint groove weld tests are:

Qualification in the 1G position qualifies for direct butt joints in the 1G position and for fillet welds in the 1F and 2F positions.   

Qualification in the 2G position qualifies for direct butt joints in the 1G and 2G positions and for fillet welds in the 1F and 2F positions.

Qualification in the 3G position qualifies for direct butt joints in the 1G, 2G, and 3G positions  and for fillet welds in the 1F, 2F, and 3F positions.   

Qualification in the 4G position qualifies for direct butt joints in the 1G and 4G positions and for fillet welds in the 1F, 2F, and 4F positions.   

I'm at work and as I compile my response, I've had a lot of interruptions.  Hopefully I haven't left anything out or made any errors.