Because you mention that your son is at a Montessori, and assuming it's a traditional Montessori (and not a school using the name without the method), I would be careful to teach place value and addition, and explain the difference, with a consideration for how the Montessori method teaches it. For the most part I would talk to the guides as you have and ask them how they approach it; this has worked very well for me with my son, who's in a Montessori (entering Lower Elementary this year). Teaching in a consistent manner with how he's learning will make it much easier for him, both to learn from you and to translate the learning to the next school year.
The below describes how our Montessori teaches the concept, which may be different from yours (there are multiple schools of Montessori teaching), though this is the most common and most similar to the traditional methods of Maria Montessori. Your child can likely tell you if he uses these methods at school, and at 4.5 he should have been exposed to this already most likely (depending on his ability level, but I believe all of the 4 year olds in my son's school had already learned this).
Place value for Montessori is most commonly taught using the Golden Beads, which is part of the Montessori method's focus on tactile methods of learning mathematics. You have single beads for 1s, strips of 10 beads for 10s, squares of 100 beads for 100s, and cubes of 1000 beads for 1000. This helps the child understand how to count a large number - 1852 is one thousand, 8 hundreds, 5 tens, 2 ones.
The next step then is Symbols, which is how this is translated into the decimal number system. Here instead of beads they have number cards - 1s cards, 10s cards, 100s cards, 1000s cards - and they get out the right cards for the number. 1 1000 8 100s 5 10s 2 1s for 1852. (The cards have the actual number on them - so 8 100s means you have 100, 200, 300, 400, 500, 600, 700, 800.)
Then they combine them in Formation of numbers, which takes a set of golden beads AND a set of symbol cards, and takes a golden beads result and translates it into a number with the symbol cards.
This is fairly straightforward, and something your son will naturally learn at the Montessori. If he stays through Kindergarten at least, he'll likely continue to use these golden beads for addition, subtraction, multiplication, and perhaps even division - my son used all four, and it became a nice link to a new concept using an old method.
Even if you don't have these materials, you likely can mimic some of them for lessons at home if he's asking for them. Maybe not the golden beads, but the symbol cards are easy to make.
If he's then interested in addition, these allow for easy teaching of addition also - as you can use either of these methods to learn it. Just keep the addition to small numbers to start with (numbers that don't require carrying), you can have him add things like 1432 + 2136 very easily using the symbol cards - just get out the first set of symbol cards (1432) and then for each place count out however many more you need.
I would definitely ask him which of these he's done already. At 4-4.5 I would expect at least some of this to have already been covered for many children, so you can skip to the point he's at and let him practice with his current level or explore a bit of the next level. If your montessori guides are available over the summer for questions, it may be worth asking them also for tips.