I've tried multiple orders on 3334 and 3333 and it's always stuck at 4000 diff  

Perhaps there is a way to add another port with a higher diff for the hashpower
Not possible without restarting the pool. I can add a custom proxy port but that will lose your workernames and combine them all into one username only. Will that suffice?
This was my first time experimenting with the hash power. I thought it would be fun but I don't want you to go out of your way for little old me.
But I think for a potential payout it's worth adding the ability during the next maintenance downtime perhaps.
NiceHash has a lot of power for SHA256 and they are very cheap. I also wanted to start with NiceHash power to Your website. Nothing happened. Minimal difficulty for the share on NiceHash SHA-256 is d=262144. And your website says that his difficulty is 4000. "Pool difficulty too low (provided=4000, minimum=65532)". After the procedure of verification of Your pool NiceHash said 
"Your pool is shown as incompatible therefore we encourage you to contact pool operator to make sure that the pool is using minimal/starting pool share difficulty which is compatible with our service (and thus compatible with today's miners), please send them link to our FAQ on this topic: 
https://www.nicehash.com/index.jsp?p=faq#faqb3". Do You can achieve compatible with Nicehash on your pool.