SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE TO

(Rule 14d-100)

Tender Offer Statement under

Section 14(d)(1) or 13(e)(1) of the Securities Exchange Act of 1934

(Amendment No. 4)

CYREN LTD.

(Name of Subject Company (Issuer))

WP XII INVESTMENTS B.V.

WP XII INVESTMENTS COÖPERATIEF U.A.

WARBURG PINCUS (CALLISTO) PRIVATE EQUITY XII (CAYMAN), L.P.

WARBURG PINCUS (EUROPA) PRIVATE EQUITY XII (CAYMAN), L.P.

WARBURG PINCUS (GANYMEDE) PRIVATE EQUITY XII (CAYMAN), L.P.

WARBURG PINCUS PRIVATE EQUITY XII-B (CAYMAN), L.P.

WARBURG PINCUS PRIVATE EQUITY XII-D (CAYMAN), L.P.

WARBURG PINCUS PRIVATE EQUITY XII-E (CAYMAN), L.P.

WARBURG PINCUS XII PARTNERS (CAYMAN), L.P.

WP XII PARTNERS (CAYMAN), L.P.

(Name of Filing Person (Offeror))

ORDINARY SHARES, NOMINAL VALUE NIS 0.15 PER SHARE

(Title of Class of Securities)

M26895108

(CUSIP Number of Class of Securities)

WP XII Investments B.V.

c/o Warburg Pincus LLC

450 Lexington Avenue

New York, NY 10017

Telephone: (212) 878-0600

(Name, address and telephone numbers of person authorized to receive

notices and communications on behalf of filing persons)

With copies to:

CALCULATION OF FILING FEE

Check the appropriate boxes below to designate any transaction to which the statement relates:

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

This Amendment No. 4 (this “Amendment”) amends and supplements the Tender Offer Statement on Schedule TO initially filed with the United States Securities and Exchange Commission (the “SEC”) on November 20, 2017, as amended by Amendment No. 1 filed with the SEC on November 28, 2017, as further amended by Amendment No. 2 filed with the SEC on December 6, 2017, as further amended by Amendment No. 3 filed with the SEC on December 14, 2017 (together with any other amendments and supplements thereto, the “Schedule TO”) by WP XII Investments B.V., a private limited liability company organized under the laws of the Netherlands (“WP XII BV”), which is a wholly owned subsidiary of WP XII Investments Coöperatief U.A., a company incorporated in The Netherlands (“WP XII Investments Coöperatief”), which is jointly owned by (i) Warburg Pincus (Callisto) Private Equity XII (Cayman), L.P., a Cayman Islands exempted limited partnership (“WP XII Callisto”), (ii) Warburg Pincus (Europa) Private Equity XII (Cayman), L.P., a Cayman Islands exempted limited partnership (“WP XII Europa”), (iii) Warburg Pincus (Ganymede) Private Equity XII (Cayman), L.P., a Cayman Islands exempted limited partnership (“WP XII Ganymede”), (iv) Warburg Pincus Private Equity XII-B (Cayman), L.P., a Cayman Islands exempted limited partnership (“WP XII-B”), (v) Warburg Pincus Private Equity XII-D (Cayman), L.P., a Cayman Islands exempted limited partnership (“WP XII-D”), (vi) Warburg Pincus Private Equity XII-E (Cayman), L.P., a Cayman Islands exempted limited partnership (“WP XII-E”), (vii) Warburg Pincus XII Partners (Cayman), L.P., a Cayman Islands exempted limited partnership (“Warburg Pincus XII Partners”) and (viii) WP XII Partners (Cayman), L.P., a Cayman Islands exempted limited partnership (“WP XII Partners,” and together with WP XII Callisto, WP XII Europa, WP XII Ganymede, WP XII-B, WP XII-D, WP XII-E and Warburg Pincus XII Partners, the “WP XII Funds”; the WP XII Funds, collectively with WP XII BV and WP XII Investments Coöperatief, the “Bidder”) and

relates to the offer by the Bidder to purchase 31,265,358 outstanding ordinary shares, nominal (par) value NIS 0.15 per share (the “Shares”), of Cyren Ltd. (“Cyren”), at $2.50 per Share, net to the seller in cash, less any applicable withholding taxes, and without interest, upon the terms of, and subject to the conditions to, the Offer to Purchase, dated November 20, 2017 (the “Offer to Purchase”) and the related Letter of Transmittal, copies of which are attached to the Schedule TO as Exhibits (a)(1)(A) and (a)(1)(B), respectively (which, together with any amendments or supplements thereto, constitute the “Offer”).

This Schedule TO is intended to satisfy the reporting requirements of Rule 14d-1 under the Securities Exchange Act of 1934, as amended. Except as specifically provided herein, the information contained in the Schedule TO, the Offer to Purchase, and the Letter of Transmittal remains unchanged and this Amendment does not modify any of the information previously reported on the Schedule TO. This Amendment should be read in conjunction with the Schedule TO, the Offer to Purchase, and the Letter of Transmittal.

The information set forth in Section 5 (“Certain Material U.S. Federal Income Tax and Israeli Income Tax Consequences”) of the Offer to Purchase is hereby amended and supplemented by adding the following paragraph after the last paragraph under such heading:

“On December 17, 2017, the Israeli Tax Authority granted approval with respect to the Israeli withholding tax rates applicable to shareholders as a result of the sale of Cyren shares pursuant to the offer. The approval provides, among other things, that payments to registered shareholders will be subject to Israeli withholding tax, unless such tendering shareholder provides the paying agent with an exemption certificate issued by the Israeli Tax Authority.”

Items 1 through 9 and 11

Items 1 through 9 and 11 of the Schedule TO, which incorporate by reference the information contained in the Offer to Purchase, are hereby amended and supplemented as follows:

(1) “On December 19, 2017, the Bidder issued a press release announcing that all of the conditions to the Offer have been satisfied and that it is commencing the Additional Offer Period. A copy of the press release is attached hereto as Exhibit (a)(5)(F) and is incorporated herein by reference.”

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SIGNATURES

After due inquiry and to the best of the knowledge and belief of the undersigned, the undersigned certify that the information set forth in this statement is true, complete and correct.

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Dated: December 19, 2017

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EXHIBIT INDEX

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