Write it the way you'd say it

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I kept your own phrases ("no choice", "not good news for anyone", "call me directly") so it still sounds like you.
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What I changed:

• The news now comes first. Your old opening ("I wanted to let you know...") delayed it, and customers will be looking for the 4% and the date.
• Split the one long sentence into short ones, so the 4% and 1 January don't get buried.
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Good, I'll leave the number alone. Here's the whole letter with a warmer end. The change is that you now come first and the rep is the extra option, not the other way round.

Dear customer,

Our prices will go up by 4% from 1 January. Our main supplier put their prices up from 1 December, and we have no choice but to pass some of that on.

We've kept the rise as low as we can, and we've already absorbed some of the increase ourselves over the last two months. I know this isn't good news for anyone.

If you have any questions, please call me directly. I'm happy to talk it through. Your sales rep can help too.

We'll keep giving you the best service we can, and thank you for your business.

Kind regards,
Sam Carter
Northfield Supply

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If "I'm happy to talk it through" isn't how you'd say it, swap in your own words. It's the one line here I added rather than kept from your draft.