Replace input field by dropdown using the Product Open Pricing plugin

functions.php

<?php
// Remove default input
add_filter( 'option_' . 'alg_wc_product_open_pricing_frontend_template', 'popwc_remove_default_input' , 10, 2 );
function popwc_remove_default_input($value){
	if(!is_admin()){
		$value='';
	}
	return $value;
}

// Add custom input
add_action( 'woocommerce_before_add_to_cart_button', 'popwc_add_input_field_to_frontend' , PHP_INT_MAX );
function popwc_add_input_field_to_frontend(){
	global $product;
	$product_id = $product->get_id();
	$post_meta = get_post_meta($product_id,'custom_field');
	?>
	<select id="alg_open_price" name="alg_open_price">
		<option>4</option>
		<option>6</option>
	</select>
	<?php	
}

Product Open Pricing plugin