A C4 envelope holds an A4 sheet flat, without folding. Sending a marketing letter in a C4 envelope makes it arrive looking like an important document, which suits formal notices, proposals and premium offers where first impressions matter.
Overview
Standard A4 letters are folded to fit a smaller envelope. A C4 envelope measures 229 × 324 mm, just larger than A4, so the page arrives flat and uncreased. That small difference changes how the mailing is perceived: large flat envelopes are associated with contracts, certificates and official correspondence.
At AutoLetters, C4 is an add-on to the A4 letter rather than a product of its own. You design the A4 letter as normal, then choose the C4 option when you set up the campaign. The price shown during planning includes the add-on, printing and UK postage.
Because a flat page is also easier to keep, file or pin up, C4 suits letters the reader may want to refer back to, such as price lists, renewal terms or an invitation with details.
When to use it
How to start, step by step
- Decide whether flat matters. If the letter is short and promotional, a folded A4 letter is usually enough. Choose C4 when the impression or the content benefits from a flat page.
- Design to the A4 letter specification. Use the A4 letter artwork measurements on the product page, including the safe zone and the address area.
- Select the C4 add-on during planning. Tell Lenny you want the letter posted flat; the quote updates to include the add-on.
- Check the print file. Confirm the address area is clear and positioned correctly before you approve.
- Track the result. Use a reference code so you can compare C4 mailings against folded letters if you test both.
Which format to choose
| Use | Format | Why | From, per item |
|---|---|---|---|
| Formal letter posted flat | Letter — A4 | Choose the C4 add-on at checkout; the letter is the base product. | £1.90 |
| Premium short message | Postcard — A5 enveloped | Enveloped and card-weight, for briefer copy. | £2.20 |
| Brochure-style content | A3 to A5 Enveloped | More pages and images in an envelope. | £2.45 |
Prices come from the live AutoLetters price list. They include printing and UK postage, are the same at any quantity, and have no VAT added because AutoLetters is not VAT registered.
Worked example
Illustrative scenario, not a customer case study. An independent financial adviser posts a year-end planning letter to 300 existing clients. The base letter cost is shown below; the C4 add-on is priced on top during planning, so the final quote will be higher.
300 × Letter — A4 at £1.90 each = £570.00, printed and posted.
Response depends on your list, offer and timing, so we do not quote response rates. Measure your own with a unique code or web address.
Mistakes to avoid
- Paying for C4 on short promotional copy that would work folded.
- Forgetting that C4 is an add-on, so the final quote is higher than the base letter price.
- Placing important content where the address sits.
- Not testing C4 against folded letters when the cost difference matters to you.
Preparing artwork that prints correctly
Every format has a trim size, a bleed area and a safe zone. Extend background colours and images into the bleed so no white edge appears after cutting, and keep text, logos and QR codes inside the safe zone. Use images at 300 DPI at printed size, and leave the address area clear, because the printer adds the recipient's address there. Each product page lists the exact supplier measurements, and Lenny checks uploaded artwork against them before you approve it.
Building a list you can use
You can send to a list you already hold or to an audience of UK addresses built around a postcode. If you use your own list, include only people you have a lawful basis to contact by post, remove anyone who has asked not to hear from you, and keep a record of where the data came from. The Information Commissioner's Office publishes guidance on direct marketing under UK GDPR. Check that every address is complete and current: incomplete addresses cost money and do not arrive.
Measuring the result
Decide how you will count responses before you send. A unique discount code, a short web address, a dedicated phone number or a simple “how did you hear about us?” question all work. Divide the campaign cost by the number of responses to get a cost per response, then compare it with your other marketing. Where you can, hold back a small group from the mailing, so you can see how many of them would have responded anyway.
How AutoLetters handles it
Lenny, the AutoLetters AI campaign specialist, helps you choose a format, drafts the wording and artwork to the printer's exact measurements, and helps build the audience: either UK addresses around a postcode or a list you already hold. You review a print-accurate preview, then approve the exact print file. Nothing is printed or posted until you approve it.
AutoLetters sends addressed mail to UK addresses only and does not offer unaddressed door drops. Postal delivery timings are not guaranteed.
Questions
Is C4 a separate product?
No. C4 is an option on the A4 letter. The letter price shown on this page is before the add-on; your quote during planning includes it.
What size is a C4 envelope?
A C4 envelope is 229 × 324 mm and holds an unfolded A4 sheet.
Plan your campaign with Lenny
Describe what you want to promote and who should receive it. Planning is free and needs no account.
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