Ok, I'm on crack and didnt realize there'd been a reply. Sorry.

4. b3

Here's a board for reference, since everyone was asking:

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8 | BR | | BB | BQ | BK | BB | BN | BR |
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7 | BP | BP | BP | | | BP | BP | BP |
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6 | | | BN | | BP | | | |
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5 | | | | BP | | | | |
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4 | | | | | | | | |
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3 | | WP | | | | WN | WP | |
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2 | WP | | WP | WP | WP | WP | WB | WP |
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1 | WR | WN | WB | WQ | WK | | | WR |
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