5 Gallon Batch in a 4 Gallon Pot

So the story goes that I would like to brew a special batch for my buddy's Bachelor Party weekend. Typically we brew together, even tough I have brewed a lot of small batches on my own in my small apartment. We have a very good relationship.... He supplies the space and I supply the knowledge, we split the cost and enjoy the fruits of our labor together. I would like to brew a Belgian Triple that I have made with great success in the past, but it has always been 5 gallon batches. Since this brew is for a surprise I won't have the appropriate kettle to brew in. I will have to use my 4 gallon pot. My current plan right now is to use my original recipe but only collect 3.5 gallons. After the 60 minute boil is finished and cool, I would then use pre-boiled water to adjust the OG accordingly. My only concern is that I won't be able to reach a high enough OG before that when I dilute the the wort I won't reach 5 gallons before dropping below my target OG. I will probably brew this up coming so if anyone has advice or experience please share.

Do you have a second pot? I brew 5 gallon batches and split the boil into two pots.

I would recommend getting a bigger pot. Wally world has imusa aluminum pots for cheap.
If that isnt an option then brew the beer twice. Split the recipe in half and brew them as 2 beers at 2.5gal. Once the beers are done mix them in the carboy.

I second the above. Or you could just swap out the base malt for extract and do a partial boil. There's a very popular extract Tripel recipe if you search dragonmead final absolution.

I would do 2 separate boils. 2.5 gallons each. Start the second mash with like 20 minutes left to go in the 1st boil. Pitch the yeast after you transfer the first batch into the fermenter. Should add what, 1.5 hours to your brew day?