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I'd like to reserve a table for four tomorrow evening.
Literal
Tomorrow [possessive-の] evening table [object-を] 4-person-portion reserve-want [but-のですが].
A quintessential polite service request. ~たいのですが trails off with が ('but...') to leave the request open-ended and polite — a common Japanese strategy where finishing the sentence would feel too direct. ４人分 uses the suffix ~分 ('portion/share/amount for'), so 4人分 is literally 'four people's worth.' This pattern extends to ~人前 (servings) and ~日分 (days' worth).