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Given the length of time that has passed between HoU's release and now, the fourth adventure is probably another victim of the 5E focus-shift.

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  Unfortunately, the "levels 1-5" thing is essentially a fib: the first adventure is balanced for a level 3 party. A 1st level group is pretty much going to be stuck picking away at it one encounter at a time before retreating, which doesn't work very well in terms of presenting a coherent story but presumably was meant to "recapture the old school feel" or whatever the talking point was at the time.

I ran it last summer as a 6 session mini-campaign and enjoyed every minute of it. It's a great, old school dungeon crawl. I think it's important to not railroad the party into doing the quests if they don't want to. One of the best parts of Undermountain is its sheer size and complexity. By railroading them you are effectively turn a huge, interesting dungeon into a linear dungeon. Instead I let the party go whereever they want and only payed attention to the quests if they intersected with the rooms that involved the quests.

I don't know how much access to technology you have at your game table, but I ran maptools on a laptop and another laptop to display the player view map. I used a single token to show the party location and then revealed only the part of undermountain that they could see. It worked out great, they loved it.

In fact, my party loved undermountain so much they want another crack at it this summer. 

.... and yes, get the Monster Vault. It'll work with MM1, but the encounters as written are designed to work with the MV monsters so it will save you a lot of time. No need to tinker with stats or redesign encounters.