[First off: if you have an INT of 20, you have a +5 modifier, not a +4. So a 4th level wizard with an INT of 20 can prepare 9 spells, not 8. Now on to the main question.]
You may be thinking in terms of 3.5e style preparation rather than 5e.
In 5e, you don't have to match spells to spell slots when preparing them. A 4th level wizard with INT 20 can prepare 9 spells, and those are the spells you are capable of casting until you choose to prepare new ones after a long rest. You have four 1st level spell slots and three 2nd level spell slots to cast them in, which you can use to cast any of his prepared spells in any combination you want.
With 9 spells prepared and 7 spell slots, you won't actually end up using all the spells you prepare (I'm ignoring Arcane Recovery for the moment). However, you have those spells as options, so you can choose what you need as the situation demands it. If you need three False Lives during the day (two 1st, one 2nd), you can do that. If you never need False Life, you can use those slots for other spells. Its about versatility and options.
Now, as to what level the spells you prepare can be, that is entirely up to you. If you are certain that you will only ever use your 1st level slots for Shield but have no idea what you'll need to use your second level slots for, you can prepare eight 2nd level spells and one 1st level spell (but this is probably not the best idea). Alternatively, you could prepare nine 1st level spells and use the 2nd level slots to "upcast" those spells (such as getting more temporary HP out of False Life or getting more targets for Charm Person). Your only real limitation is that you can't prepare a spell of a level that you cannot cast (so a 4th level necromancer can't prepare a 3rd level spell).