I live in Philadelphia PA. Is it normal to have straw-colored lawn
patches on the lawn in the winter? These straw patches dont exist in
the grass Ive sown in the past 3 or 4 years.

That depends on your definition of normal. My tall fescue/blue grass
lawn here in NJ is still all nice and green. If most of your lawn is
green and it has been reasonably maintained, adequate fertilizer etc,
then the staw colored patches are likely some undesirable grass.
Hopefully it's not Zoysia that someone planted, as that is the classic
straw color grass. It's not suited to this environment and will go
straw color from Oct thru May. If the areas aren't too big or
numerous, your best bet may be to mark them and then kill them when the
grass is active in spring.

My lawn needs renewal but
I cannot dig the entire thing up. There are mossy patches around the
edges. Also low patches.

You usually don;t need to dig the entire thing up, unless the topsoil
is non-existent. If most of what;s there is good and the bad can be
eliminated, you can get topsoil for the low spots and overseed the
whole thing using a slice seeder. Best time by far is early Sept, next
best is early Spring. If you have a lot of problem weeds/grasses, you
can kill it off with Roundup and reseed a week later. Many times this
can be the easiest solution, especially if you have grasses/weeds that
are difficult or impossible to eradicate, grass that is disease prone,
etc. I'd also get the soil tested now and adjust the PH if needed.

I am wondering when to apply pre-emergent
weed killer and fertilizer and new seed and which new seed to use and
if I can erradicate the winter straw colored patches.

For an existing lawn, apply pre-emergent when forsythias bloom,
typically later april in your area. If you seed, then you can't use a
conventional pre-emergent, you need to use Tupersan, or similar which
is safe for new lawns. These cost a lot more and are another reason
why fall is best time for seeding. If you fertilize twice in fall,
then in spring I would apply fertilizer once, same time as
pre-emergent. Then don't fertilize again till Fall.

For new seed, in your area, unless you want a showcase lawn with higher
maintenance, I'd probably go with a dwarf tall fescue/blue grass mix.
Seedland.com has seed and some good info.