These verses seem to say that because Hashem calls Am Yisrael "Beni Bechori" (see 4:22:1), that is why Pharaoh ought to send them out to serve Him. What is the connection?
Maharal (Be'er ha'Golah, Be'er #3, p. 42): To "serve" Hashem means with Korbanos; and Bnei Yisrael are the most suitable for this task. The word Korban comes from the word "Karov;" and since only the Bnei Yisrael are called "children of Hashem," no one is closer.
Why did Hashem instruct Moshe already now to mention the last plague, the killing of the firstborn, to Pharaoh?
Rashi: To soften Pharaoh's stance, upon hearing him beginning to refuse. He warned Pharaoh already at the outset, 1 what he could expect should he refuse, on account of the severity of the punishment.
Ramban: This was not a command; He was informing Moshe that there would come a stage where Pharaoh had refused so many times that he would tell him about the impending Makas Bechoros as a result of his refusal. 2
Seforno: In fact, Makas Bechoros was the only plague that would come as a punishment, Midah k'Neged Midah 3 for refusing Hashem's firstborn to go and worship Him. The first nine plagues were to prompt Pharaoh to repent and to let Yisrael go and worship Him primarily based on the love of His goodness and in awe of His Supremacy - or at least like servants who are afraid of their master's ability to punish them. 4
Rashi: Unlike a human-being, who, afraid that his enemy will stall his plans, does not reveal them in advance, Hashem, who is All-Powerful, can reveal His plans, because nobody can prevent them from materializing, as the Pasuk states in Iyov 36:22.
Ramban: Since we do not find that Moshe told Pharaoh about Makas Bechoros until then (See 11:5). The Ramban interprets "va'Tema'en l'Shalecho" in the past tense (as a statement); as opposed to Rashi and the other commentaries, who interpret it (as a warning) in the future.
Seforno: Which the Pasuk teaches us in Iyov 34:11.
