Filed Pursuant to Rule 424(b)(3)
of the Rules and Regulations
Under the Securities Act of 1933
Registration No. 333-121255

PROSPECTUS SUPPLEMENT #4 TO PROSPECTUS DATED APRIL 14, 2005

$175,000,000

1.75% Convertible Senior Notes due August 1, 2024

and Shares of Common Stock Issuable Upon Conversion of the Notes

     This prospectus supplement supplements the prospectus dated April 14, 2005, which forms a part of the Post-Effective Amendment No. 1 to Form S-1 on Form S-3 Registration Statement (Registration No. 333-121255) that we filed with the Securities and Exchange Commission on March 25, 2005. The prospectus and this prospectus supplement relate to resales of our 1.75% Convertible Senior Notes due August 1, 2024 and shares of our common stock issuable upon conversion of the notes. The prospectus and this prospectus supplement also relate to the issuance of shares of our common stock upon conversion of the notes by holders other than the selling securityholders identified in the prospectus under “Selling Securityholders,” unless such issuance qualifies for the exemption under Section 3(a)(9) of the Securities Act of 1933, as amended, or the Securities Act.

     The prospectus is hereby supplemented to include the information in the table set forth below, which supplements the information in the “Selling Securityholders” table included in the prospectus.

     We have prepared the table set forth below based on information given to us by or on behalf of the selling securityholders identified in this prospectus supplement on or prior to March 16, 2006. Such selling securityholders, including their transferees, pledgees or donees or their successors, may from time to time offer and sell under the prospectus any or all of the notes and any shares of common stock issued upon conversion of the notes. See “Plan of Distribution” included in the prospectus. Because the selling securityholders may offer all or some portion of their notes or shares of common stock issued upon conversion of the notes, we cannot provide an estimate as to the principal amount of the notes or the number of shares of the common stock issuable upon conversion of the notes that will be held by the selling securityholders upon termination of any sales. In addition, the selling securityholders identified below may have sold, transferred or otherwise disposed of all or some portion of their notes since the date on which they provided the information regarding their notes under the prospectus or in transactions exempt from the registration requirements of the Securities Act.

     You should read this prospectus supplement in conjunction with the prospectus. This prospectus supplement updates information in the prospectus. If there is any inconsistency between the information in the prospectus and this prospectus supplement, you should rely on the information in this prospectus supplement.

Number of

Principal

Percentage of

Shares of

Number of

Amount of

Notes

Common

Shares of

Number of

Notes Owned

Outstanding

Principal

Stock Owned

Common

Shares of

Before the

Before the

Amount of

Prior to

Stock Offered

Common

Offering and

Offering and

Notes Owned

Conversion

Upon

Stock Owned

Offered for

Offered for

After the

Before the

Conversion of

After the

Selling Securityholder

Sale(1)

Sale

Offering(2)

Offering

the Notes(1)(3)

Offering(4)

Gaia Offshore Master Fund, Ltd. (5)

$

2,250,000

1.29

%

—

—

—

—

Lyxor/Gaia II Fund Ltd. (6)

$

750,000

*

—

—

—

—

The date of this prospectus supplement is March 17, 2006