Mistakes Wanted: Sell or Continue?

Hello together! I am standing on the hose and need your help. In 2015 we bought a Skoda Fabia from first possession for almost 800€ (bar), the car is from 2009. According to DAT, the value of the car is currently at just over 4000€, with Mobile comparable cars are advertised for 650€. Now to the actual fact: The car is basically good, today has got TÜV fault-free and we want to continue it for at least 4 years, because it satisfies us in all matters and will also satisfy us (Child seat fits in, etc.) I wonder, however, if it would be more useful financially to sell the car now and take the 600€ (pi times thumbs) as a down payment for a new vehicle, to finance the rest and at the end, after the financing with a vehicle, which is more value. So, which option is smarter: a) Keep the car and drive for another 4 years. Value of the car should be at 1500€ (is that too pessimistic? I mean, he would then be 12 years old) In other words, since the car is paid and belongs to us, I would pay the loss of value + necessary repairs. b) Sell the car now, invest the proceeds as a deposit for a newer vehicle (about 1200€), pay the rest (runtime depends on, between 24 and 48 everything is possible, also completely cash is conceivable) and in 4 years with a maximum of 5-6 years old vehicle. In this case, I pay the difference between deposit and Purchase price (at 600€, I say), the interest, but in the end have a newer and more valuable car. Where is my mistake of thinking? Rejoice me about illuminating answers! Best regards, Steffen